Artist view of New Horizons passing Pluto and three of its moons.. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
Now more than a year after its historic flyby of Pluto, the New Horizons spacecraft continues to speed through the Kuiper Belt. It’s currently on a beeline towards its next target of exploration, a KBO called 2014 MU69. But during its travels, New Horizons spotted another KBO, one of Pluto’s pals, Quaoar.
This animated sequence shows composite images of the Kuiper Belt object Quaoar, taken by New Horizons’ Long Range Reconnaissance Imager (LORRI). Click on the image to animate. Credit: NASA/JHUAPL/SwRI.
When these images were taken (in July 2016), Quaoar was approximately 4 billion miles (6.4 billion kilometers) from the Sun and 1.3 billion miles (2.1 billion kilometers) from New Horizons.
The animated sequence, above, (click the image if it isn’t animating in your browser) shows composite images taken by New Horizons’ Long Range Reconnaissance Imager (LORRI) at four different times over July 13-14: “A” on July 13 at 02:00 Universal Time; “B” on July 13 at 04:08 UT; “C” on July 14 at 00:06 UT; and “D” on July 14 at 02:18 UT. The New Horizons team explained that each composite includes 24 individual LORRI images, providing a total exposure time of 239 seconds and making the faint object easier to see.
Quaoar ( pronounced like “Kwa-war”) is about 690 miles or 1,100 kilometers in diameter, about half the size of Pluto. It was discovered on June 4, 2002 by astronomers Mike Brown and Chad Trujillo from Caltech, and at the time of its discovery, it was the largest object found in the Solar System since the discovery of Pluto. Quaoar’s discovery was one of the things that spurred the discussion of whether Pluto should continue to be classified as a planet or not.
But Quaoar is an interesting object in its own right and the New Horizons team said the oblique views of it that New Horizons can see – where LORRI sees only a portion of Quaoar’s illuminated surface — is very different from the nearly fully illuminated view of it that is visible from Earth. Comparing Quaoar from the two very different perspectives gives mission scientists a valuable opportunity to study the light-scattering properties of Quaoar’s surface.
If you’re thinking, “Why don’t we send a mission to Quaoar, or Sedna or Eris?” you aren’t alone. New Horizons team member Alex Parker has obviously been thinking about it. Parker tweeted that for a New Horizons-like mission it would take about 13 and a half years to reach Quaoar if it could be launched in December 2016. “Otherwise, we have to wait another 11 years for the next Jupiter assist window,” he said.
Um, NASA, can we put this on the schedule for 2027?
In the meantime, the images and data that New Horizons gathered during the Pluto flyby in July 2015 are still trickling back to Earth. The image below is a stunning view of Pluto’s methane snowcaps, visible at the terminator, showing the region north of Pluto’s dark equatorial band informally named Cthulhu Regio, and southwest of the vast nitrogen ice plains informally named Sputnik Planitia. This image was taken about 45 minutes before New Horizons’ closest approach to Pluto on July 14, 2015.
This area is south of Pluto’s dark equatorial band informally named Cthulhu Regio, and southwest of the vast nitrogen ice plains informally named Sputnik Planitia. North is at the top; in the western portion of the image, a chain of bright mountains extends north into Cthulhu Regio. New Horizons compositional data indicate the bright snowcap material covering these mountains isn’t water, but atmospheric methane that has condensed as frost onto these surfaces at high elevation. Between some mountains are sharply cut valleys – indicated by the white arrows. These valleys are each a few miles across and tens of miles long. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.
See all of the latest photos sent back from our robot in the outer reaches of our Solar System at the New Horizons website.
An illustration showing the 8 planets of the Solar System to scale Credit: NASA
At one time, humans believed that the Earth was the center of the Universe; that the Sun, Moon, planets and stars all revolved around us. It was only after centuries of ongoing observations and improved instrumentation that astronomers came to understand that we are in fact part a larger system of planets that revolve around the Sun. And it has only been within the last century that we’ve come to understand just how big our Solar System is.
And even now, we are still learning. In the past few decades, the total number of celestial bodies and moons that are known to orbit the Sun has expanded. We have also come to debate the definition of “planet” (a controversial topic indeed!) and introduced additional classifications – like dwarf planet, minor planet, plutoid, etc. – to account for new finds. So just how many planets are there and what is special about them? Let’s run through them one by one, shall we?
Mercury:
As you travel outward from the Sun, Mercury is the closest planet. It orbits the Sun at an average distance of 58 million km (36 million mi). Mercury is airless, and so without any significant atmosphere to hold in the heat, it has dramatic temperature differences. The side that faces the Sun experiences temperatures as high as 420 °C (788 °F), and then the side in shadow goes down to -173 °C (-279.4 °F).
MESSENGER image of Mercury from its third flyby. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Like Venus, Earth and Mars, Mercury is a terrestrial planet, which means it is composed largely of refractory minerals such as the silicates and metals such as iron and nickel. These elements are also differentiated between a metallic core and a silicate mantle and crust, with Mercury possessing a larger-than-average core. Multiple theories have been proposed to explain this, the most widely accepted being that the impact from a planetesimal in the past blew off much of its mantle material.
Mercury is the smallest planet in the Solar System, measuring just 4879 km across at its equator. However, it is second densest planet in the Solar System, with a density of 5.427 g/cm3 – which is the second only to Earth. Because of this, Mercury experiences a gravitational pull that is roughly 38% that of Earth’s (0.38 g).
Mercury also has the most eccentric orbit of any planet in the Solar System (0.205), which means its distance from the Sun ranges from 46 to 70 million km (29-43 million mi). The planet also takes 87.969 Earth days to complete an orbit. But with an average orbital speed of 47.362 km/s, Mercury also takes 58.646 days to complete a single rotation.
Combined with its eccentric orbit, this means that it takes 176 Earth days for the Sun to return to the same place in the sky (i.e. a solar day) on Mercury, which is twice as long as a single Hermian year. Mercury also has the lowest axial tilt of any planet in the Solar System – approximately 0.027 degrees – compared to Jupiter’s 3.1 degrees, which is the second smallest.
The MESSENGER spacecraft has been in orbit around Mercury since March 2011 – but its days are numbered. Credit: NASA/JHUAPL/Carnegie Institution of Washington
Mercury has only been visited two times by spacecraft, the first being the Mariner 10 probe, which conducted a flyby of the planet back in the mid-1970s. It wasn’t until 2008 that another spacecraft from Earth made a close flyby of Mercury (the MESSENGER probe) which took new images of its surface, shed light on its geological history, and confirmed the presence of water ice and organic molecules in its northern polar region.
In summary, Mercury is made special by the fact it is small, eccentric, and varies between extremes of hot and cold. It’s also very mineral rich, and quite dense!
Venus:
Venus is the second planet in the Solar System, and is Earth’s virtual twin in terms of size and mass. With a mass of 4.8676×1024 kg and a mean radius of about 6,052 km, it is approximately 81.5% as massive as Earth and 95% as large. Like Earth (and Mercury and Mars), it is a terrestrial planet, composed of rocks and minerals that are differentiated.
But apart from these similarities, Venus is very different from Earth. Its atmosphere is composed primarily of carbon dioxide (96%), along with nitrogen and a few other gases. This dense cloud cloaks the planet, making surface observation very difficult, and helps heat it up to 460 °C (860 °F). The atmospheric pressure is also 92 times that of Earth’s atmosphere, and poisonous clouds of carbon dioxide and sulfuric acid rain are commonplace.
Venus’ similarity in size and mass has led to it being called “Earth’s sister planet’. Credit: NASA/JPL/Magellan
Venus orbits the Sun at an average distance of about 0.72 AU (108 million km; 67 million mi) with almost no eccentricity. In fact, with its farthest orbit (aphelion) of 0.728 AU (108,939,000 km) and closest orbit (perihelion) of 0.718 AU (107,477,000 km), it has the most circular orbit of any planet in the Solar System. The planet completes an orbit around the Sun every 224.65 days, meaning that a year on Venus is 61.5% as long as a year on Earth.
When Venus lies between Earth and the Sun, a position known as inferior conjunction, it makes the closest approach to Earth of any planet, at an average distance of 41 million km. This takes place, on average, once every 584 days, and is the reason why Venus is the closest planet to Earth. The planet completes an orbit around the Sun every 224.65 days, meaning that a year on Venus is 61.5% as long as a year on Earth.
Unlike most other planets in the Solar System, which rotate on their axes in an counter-clockwise direction, Venus rotates clockwise (called “retrograde” rotation). It also rotates very slowly, taking 243 Earth days to complete a single rotation. This is not only the slowest rotation period of any planet, it also means that a single day on Venus lasts longer than a Venusian year.
Venus’ atmosphere is also known to experience lightning storms. Since Venus does not experience rainfall (except in the form of sulfuric acid), it has been theorized that the lightning is being caused by volcanic eruptions. Several spacecraft have visited Venus, and a few landers have even made it to the surface to send back images of its hellish landscape. Even though there were made of metal, these landers only survived a few hours at best.
Venus is made special by the fact that it is very much like Earth, but also radically different. It’s thick atmosphere could crush a living being, its heat could melt lead, and its acid rain could dissolve flesh, bone and metal alike! It also rotates very slowly, and backwards relative to the other plants.
Earth:
Earth is our home, and the third planet from the Sun. With a mean radius of 6371 km and a mass of 5.97×1024 kg, it is the fifth largest and fifth most-massive planet in the Solar System. And with a mean density of 5.514 g/cm³, it is the densest planet in the Solar System. Like Mercury, Venus and Mars, Earth is a terrestrial planet.
But unlike these other planets, Earth’s core is differentiated between a solid inner core and liquid outer core. The outer core also spins in the opposite direction as the planet, which is believed to create a dynamo effect that gives Earth its protective magnetosphere. Combined with a atmosphere that is neither too thin nor too thick, Earth is the only planet in the Solar System known to support life.
The Earth’s layers, showing the Inner and Outer Core, the Mantle, and Crust. Credit: discovermagazine.com
In terms of its orbit, Earth has a very minor eccentricity (approx. 0.0167) and ranges in its distance from the Sun between 147,095,000 km (0.983 AU) at perihelion to 151,930,000 km (1.015 AU) at aphelion. This works out to an average distance (aka. semi-major axis) of 149,598,261 km, which is the basis of a single Astronomical Unit (AU)
The Earth has an orbital period of 365.25 days, which is the equivalent of 1.000017 Julian years. This means that every four years (in what is known as a Leap Year), the Earth calendar must include an extra day. Though a single solar day on Earth is considered to be 24 hours long, our planet takes precisely 23h 56m and 4 s to complete a single sidereal rotation (0.997 Earth days).
Earth’s axis is also tilted 23.439281° away from the perpendicular of its orbital plane, which is responsible for producing seasonal variations on the planet’s surface with a period of one tropical year (365.24 solar days). In addition to producing variations in terms of temperature, this also results in variations in the amount of sunlight a hemisphere receives during the course of a year.
Earth has only a single moon: the Moon. Thanks to examinations of Moon rocks that were brought back to Earth by the Apollo missions, the predominant theory states that the Moon was created roughly 4.5 billion years ago from a collision between Earth and a Mars-sized object (known as Theia). This collision created a massive cloud of debris that began circling our planet, which eventually coalesced to form the Moon we see today.
A picture of Earth taken by Apollo 11 astronauts. Credit: NASA
What makes Earth special, you know, aside from the fact that it is our home and where we originated? It is the only planet in the Solar System where liquid, flowing water exists in abundance on its surface, has a viable atmosphere, and a protective magnetosphere. In other words, it is the only planet (or Solar body) that we know of where life can exist on the surface.
In addition, no planet in the Solar System has been studied as well as Earth, whether it be from the surface or from space. Thousands of spacecraft have been launched to study the planet, measuring its atmosphere, land masses, vegetation, water, and human impact. Our understanding of what makes our planet unique in our Solar System has helped in the search for Earth-like planets in other systems.
Mars:
The fourth planet from the Sun is Mars, which is also the second smallest planet in the Solar System. It has a radius of approximately 3,396 km at its equator, and 3,376 km at its polar regions – which is the equivalent of roughly 0.53 Earths. While it is roughly half the size of Earth, it’s mass – 6.4185 x 10²³ kg – is only 0.151 that of Earth’s. It’s density is also lower than Earths, which leads to it experiencing about 1/3rd Earth’s gravity (0.376 g).
It’s axial tilt is very similar to Earth’s, being inclined 25.19° to its orbital plane (Earth’s axial tilt is just over 23°), which means Mars also experiences seasons. Mars has almost no atmosphere to help trap heat from the Sun, and so temperatures can plunge to a low of -140 °C (-220 °F) in the Martian winter. However, at the height of summer, temperatures can get up to 20 °C (68 °F) during midday at the equator.
However, recent data obtained by the Curiosity rover and numerous orbiters have concluded that Mars once had a denser atmosphere. Its loss, according to data obtained by NASA’s Mars Atmosphere and Volatile Evolution (MAVEN), the atmosphere was stripped away by solar wind over the course of a 500 million year period, beginning 4.2 billion years ago.
At its greatest distance from the Sun (aphelion), Mars orbits at a distance of 1.666 AUs, or 249.2 million km. At perihelion, when it is closest to the Sun, it orbits at a distance of 1.3814 AUs, or 206.7 million km. At this distance, Mars takes 686.971 Earth days, the equivalent of 1.88 Earth years, to complete a rotation of the Sun. In Martian days (aka. Sols, which are equal to one day and 40 Earth minutes), a Martian year is 668.5991 Sols.
Like Mercury, Venus, and Earth, Mars is a terrestrial planet, composed mainly of silicate rock and metals that are differentiated between a core, mantle and crust. The red-orange appearance of the Martian surface is caused by iron oxide, more commonly known as hematite (or rust). The presence of other minerals in the surface dust allow for other common surface colors, including golden, brown, tan, green, and others.
Although liquid water cannot exist on Mars’ surface, owing to its thin atmosphere, large concentrations of ice water exist within the polar ice caps – Planum Boreum and Planum Australe. In addition, a permafrost mantle stretches from the pole to latitudes of about 60°, meaning that water exists beneath much of the Martian surface in the form of ice water. Radar data and soil samples have confirmed the presence of shallow subsurface water at the middle latitudes as well.
MSL Curiosity selfie on the surface of Mars. Image: NASA/JPL/Cal-Tech
Mars has two tiny asteroid-sized moons: Phobos and Deimos. Because of their size and shape, the predominant theory is that Mars acquired these two moons after they were kicked out of the Asteroid Belt by Jupiter’s gravity.
Mars has been heavily studied by spacecraft. There are multiple rovers and landers currently on the surface and a small fleet of orbiters flying overhead. Recent missions include the Curiosity Rover, which gathered ample evidence on Mars’ water past, and the groundbreaking discovery of finding organic molecules on the surface. Upcoming missions include NASA’s InSight lander and the Exomars rover.
Hence, Mars’ special nature lies in the fact that it also is terrestrial and lies within the outer edge of the Sun’s habitable zone. And whereas it is a cold, dry place today, it once had an thicker atmosphere and plentiful water on its surface.
Jupiter:
Mighty Jupiter is the fouth planet for our Sun and the biggest planet in our Solar System. Jupiter’s mass, volume, surface area and mean circumference are 1.8981 x 1027 kg, 1.43128 x 1015 km3, 6.1419 x 1010 km2, and 4.39264 x 105 km respectively. To put that in perspective, Jupiter diameter is roughly 11 times that of Earth, and 2.5 times the mass of all the other planets in the Solar System combined.
Jupiter has spectacular aurora, such as this view captured by the Hubble Space Telescope. Credit: NASA, ESA, and J. Nichols (University of Leicester)
But, being a gas giant, it has a relatively low density – 1.326 g/cm3 – which is less than one quarter of Earth’s. This means that while Jupiter’s volume is equivalent to about 1,321 Earths, it is only 318 times as massive. The low density is one way scientists are able to determine that it is made mostly of gases, though the debate still rages on what exists at its core (see below).
Jupiter orbits the Sun at an average distance (semi-major axis) of 778,299,000 km (5.2 AU), ranging from 740,550,000 km (4.95 AU) at perihelion and 816,040,000 km (5.455 AU) at aphelion. At this distance, Jupiter takes 11.8618 Earth years to complete a single orbit of the Sun. In other words, a single Jovian year lasts the equivalent of 4,332.59 Earth days.
However, Jupiter’s rotation is the fastest of all the Solar System’s planets, completing a rotation on its axis in slightly less than ten hours (9 hours, 55 minutes and 30 seconds to be exact). Therefore, a single Jovian year lasts 10,475.8 Jovian solar days. This orbital period is two-fifths that of Saturn, which means that the two largest planets in our Solar System form a 5:2 orbital resonance.
Much like Earth, Jupiter experiences auroras near its northern and southern poles. But on Jupiter, the auroral activity is much more intense and rarely ever stops. The intense radiation, Jupiter’s magnetic field, and the abundance of material from Io’s volcanoes that react with Jupiter’s ionosphere create a light show that is truly spectacular.
The Juno spacecraft isn’t the first one to visit Jupiter. Galileo went there in the mid 90’s, and Voyager 1 snapped a nice picture of the clouds on its mission. Credit: NASA
Jupiter also experiences violent weather patterns. Wind speeds of 100 m/s (360 km/h) are common in zonal jets, and can reach as high as 620 kph (385 mph). Storms form within hours and can become thousands of km in diameter overnight. One storm, the Great Red Spot, has been raging since at least the late 1600s. The storm has been shrinking and expanding throughout its history; but in 2012, it was suggested that the Giant Red Spot might eventually disappear.
Jupiter is composed primarily of gaseous and liquid matter. It is the largest of the gas giants, and like them, is divided between a gaseous outer atmosphere and an interior that is made up of denser materials. It’s upper atmosphere is composed of about 88–92% hydrogen and 8–12% helium by percent volume of gas molecules, and approx. 75% hydrogen and 24% helium by mass, with the remaining one percent consisting of other elements.
The interior contains denser materials, such that the distribution is roughly 71% hydrogen, 24% helium and 5% other elements by mass. It is believed that Jupiter’s core is a dense mix of elements – a surrounding layer of liquid metallic hydrogen with some helium, and an outer layer predominantly of molecular hydrogen. The core has also been described as rocky, but this remains unknown as well.
Jupiter has been visited by several spacecraft, including NASA’s Pioneer 10 and Voyager spacecraft in 1973 and 1980, respectively; and by the Cassini and New Horizons spacecraft more recently. Until the recent arrival of Juno, only the Galileo spacecraft has ever gone into orbit around Jupiter, and it was crashed into the planet in 2003 to prevent it from contaminating one of Jupiter’s icy moons.
Illustration of Jupiter and the Galilean satellites. Credit: NASA
In short, Jupiter is massive and has massive storms. But compared to the planets of the inner Solar System, is it significantly less dense. Jupiter also has the most moons in the Solar System, with 67 confirmed and named moons orbiting it. But it is estimated that as many as 200 natural satellites may exist around the planet. Little wonder why this planet is named after the king of the gods.
Saturn:
Saturn is the second largest planet in the Solar System. With a mean radius of 58232±6 km, it is approximately 9.13 times the size of Earth. And at 5.6846×1026 kg, it is roughly 95.15 as massive. However, since it is a gas giant, it has significantly greater volume – 8.2713×1014 km3, which is equivalent to 763.59 Earths.
The sixth most distant planet, Saturn orbits the Sun at an average distance of 9 AU (1.4 billion km; 869.9 million miles). Due to its slight eccentricity, the perihelion and aphelion distances are 9.022 (1,353.6 million km; 841.3 million mi) and 10.053 AU (1,513,325,783 km; 940.13 million mi), on average respectively.
With an average orbital speed of 9.69 km/s, it takes Saturn 10,759 Earth days to complete a single revolution of the Sun. In other words, a single Cronian year is the equivalent of about 29.5 Earth years. However, as with Jupiter, Saturn’s visible features rotate at different rates depending on latitude, and multiple rotation periods have been assigned to various regions.
This portrait looking down on Saturn and its rings was created from images obtained by NASA’s Cassini spacecraft on Oct. 10th, 2013. Credit: NASA/JPL-Caltech/Space Science Institute/G. Ugarkovic
As a gas giant, Saturn is predominantly composed of hydrogen and helium gas. With a mean density of 0.687 g/cm3, Saturn is the only planet in the Solar System that is less dense than water; which means that it lacks a definite surface, but is believed to have a solid core. This is due to the fact that Saturn’s temperature, pressure, and density all rise steadily toward the core.
Standard planetary models suggest that the interior of Saturn is similar to that of Jupiter, having a small rocky core surrounded by hydrogen and helium with trace amounts of various volatiles. This core is similar in composition to the Earth, but more dense due to the presence of metallic hydrogen, which as a result of the extreme pressure.
As a gas giant, the outer atmosphere of Saturn contains 96.3% molecular hydrogen and 3.25% helium by volume. Trace amounts of ammonia, acetylene, ethane, propane, phosphine and methane have been also detected in Saturn’s atmosphere. Like Jupiter, it also has a banded appearance, but Saturn’s bands are much fainter and wider near the equator.
On occasion, Saturn’s atmosphere exhibits long-lived ovals that are thousands of km wide, similar to what is commonly observed on Jupiter. Whereas Jupiter has the Great Red Spot, Saturn periodically has what’s known as the Great White Spot (aka. Great White Oval). This unique but short-lived phenomenon occurs once every Saturnian year, roughly every 30 Earth years, around the time of the northern hemisphere’s summer solstice.
The huge storm churning through the atmosphere in Saturn’s northern hemisphere overtakes itself as it encircles the planet in this true-color view from NASA’s Cassini spacecraft. Image credit: NASA/JPL-Caltech/SSI
The persisting hexagonal wave pattern around the north pole was first noted in the Voyager images. The sides of the hexagon are each about 13,800 km (8,600 mi) long (which is longer than the diameter of the Earth) and the structure rotates with a period of 10h 39m 24s, which is assumed to be equal to the period of rotation of Saturn’s interior.
The south pole vortex, meanwhile, was first observed using the Hubble Space Telescope. These images indicated the presence of a jet stream, but not a hexagonal standing wave. These storms are estimated to be generating winds of 550 km/h, are comparable in size to Earth, and believed to have been going on for billions of years. In 2006, the Cassini space probe observed a hurricane-like storm that had a clearly defined eye. Such storms had not been observed on any planet other than Earth – even on Jupiter.
Of course, the most amazing feature of Saturn is its rings. These are made of particles of ice ranging in size from a grains of sand to the size of a car. Some scientists think the rings are only a few hundred million years old, while others think they could be as old as the Solar System itself.
Saturn has been visited by spacecraft 4 times: Pioneer 11, Voyager 1 and 2 were just flybys, but Cassini has actually gone into orbit around Saturn and has captured thousands of images of the planet and its moons. And speaking of moons, Saturn has a total of 62 moons discovered (so far), though estimates indicate that it might have as many as 150.
A collage of Saturn (bottom left) and some of its moons: Titan, Enceladus, Dione, Rhea and Helene. Credit: NASA/JPL/Space Science Institute
So like Jupiter, Saturn is a massive gas giant that experiences some very interesting weather patterns. It also has lots of moons and has a very low density. But what really makes Saturn stand out is its impressive ring system. Whereas every gas and ice giant has one, Saturn’s is visible to the naked eye and very beautiful to behold!
Uranus:
Next comes Uranus, the seventh planet from the Sun. With a mean radius of approximately 25,360 km and a mass of 8.68 × 1025 kg, Uranus is approximately 4 times the sizes of Earth and 63 times its volume. However, as a gas giant, its density (1.27 g/cm3) is significantly lower; hence, it is only 14.5 as massive as Earth.
The variation of Uranus’ distance from the Sun is also greater than that any other planet (not including dwarf planets or plutoids). Essentially, the gas giant’s distance from the Sun varies from 18.28 AU (2,735,118,100 km) at perihelion to 20.09 AU (3,006,224,700 km) at aphelion. At an average distance of 3 billion km from the Sun, it takes Uranus roughly 84 years (or 30,687 days) to complete a single orbit of the Sun.
The standard model of Uranus’s structure is that it consists of three layers: a rocky (silicate/iron–nickel) core in the center, an icy mantle in the middle and an outer envelope of gaseous hydrogen and helium. Much like Jupiter and Saturn, hydrogen and helium account for the majority of the atmosphere – approximately 83% and 15% – but only a small portion of the planet’s overall mass (0.5 to 1.5 Earth masses).
Uranus as seen through the automated eyes of Voyager 2 in 1986. (Credit: NASA/JPL)
The third most abundant element is methane ice (CH4), which accounts for 2.3% of its composition and which accounts for the planet’s aquamarine or cyan coloring. Trace amounts of various hydrocarbons are also found in the stratosphere of Uranus, which are thought to be produced from methane and ultraviolent radiation-induced photolysis. They include ethane (C2H6), acetylene (C2H2), methylacetylene (CH3C2H), and diacetylene (C2HC2H).
In addition, spectroscopy has uncovered carbon monoxide and carbon dioxide in Uranus’ upper atmosphere, as well as the presence icy clouds of water vapor and other volatiles, such as ammonia and hydrogen sulfide. Because of this, Uranus and Neptune are considered a distinct class of giant planet – known as “Ice Giants” – since they are composed mainly of heavier volatile substances.
The rotational period of the interior of Uranus is 17 hours, 14 minutes. As with all giant planets, its upper atmosphere experiences strong winds in the direction of rotation. Hence its weather systems are also broken up into bands that rotate around the planet, which are driven by internal heat rising to the upper atmosphere.
As a result, winds on Uranus can reach up to 900 km/h (560 mph), creating massive storms like the one spotted by the Hubble Space Telescope in 2012. Similar to Jupiter’s Great Red Spot, this “Dark Spot” was a giant cloud vortex that measured 1,700 kilometers by 3,000 kilometers (1,100 miles by 1,900 miles).
Huge storms on Uranus were spotted by the Keck Observatory on Aug. 5 and Aug. 6, 2014. Credit: Imke de Pater (UC Berkeley), Pat Fry (University of Wisconsin), Keck Observatory
One unique feature of Uranus is that it rotates on its side. Whereas all of the Solar System’s planets are tilted on their axes to some degree, Uranus has the most extreme axial tilt of 98°. This leads to the radical seasons that the planet experiences, not to mention an unusual day-night cycle at the poles. At the equator, Uranus experiences normal days and nights; but at the poles, each experience 42 Earth years of day followed by 42 years of night.
Uranus was the first planet to be discovered with a telescope; it was first recognized as a planet in 1781 by William Herschel. Beyond Earth-based observations, only one spacecraft (Voyager 2) has ever studied Uranus up close. It passed by the planet in 1986, and captured the first close images. Uranus has 27 known moons.
Uranus’ special nature comes through in its natural beauty, its intense weather, its ring system and its impressive array of moons. And it’s compositions, being an “ice” giant, is what gives its aquamarine color. But perhaps mist interesting is its sideways rotation, which is unique among the Solar planets.
Neptune:
Neptune is the 8th and final planet in the Solar System, orbiting the Sun at a distance of 29.81 AU (4.459 x 109 km) at perihelion and 30.33 AU (4.537 x 109 km) at aphelion. With a mean radius of 24,622 ± 19 km, Neptune is the fourth largest planet in the Solar System and four times as large as Earth. But with a mass of 1.0243×1026 kg – which is roughly 17 times that of Earth – it is the third most massive, outranking Uranus.
Neptune’s system of moons and rings visualized. Credit: SETI
Neptune takes 16 h 6 min 36 s (0.6713 days) to complete a single sidereal rotation, and 164.8 Earth years to complete a single orbit around the Sun. This means that a single day lasts 67% as long on Neptune, whereas a year is the equivalent of approximately 60,190 Earth days (or 89,666 Neptunian days).
Due to its smaller size and higher concentrations of volatiles relative to Jupiter and Saturn, Neptune (much like Uranus) is often referred to as an “ice giant” – a subclass of a giant planet. Also like Uranus, Neptune’s internal structure is differentiated between a rocky core consisting of silicates and metals; a mantle consisting of water, ammonia and methane ices; and an atmosphere consisting of hydrogen, helium and methane gas.
The core of Neptune is composed of iron, nickel and silicates, with an interior model giving it a mass about 1.2 times that of Earth. The pressure at the center is estimated to be 7 Mbar (700 GPa), about twice as high as that at the center of Earth, and with temperatures as high as 5,400 K. At a depth of 7000 km, the conditions may be such that methane decomposes into diamond crystals that rain downwards like hailstones.
Because Neptune’s axial tilt (28.32°) is similar to that of Earth (~23°) and Mars (~25°), the planet experiences similar seasonal changes. Combined with its long orbital period, this means that the seasons last for forty Earth years. Also owing to its axial tilt being comparable to Earth’s is the fact that the variation in the length of its day over the course of the year is not any more extreme than it on Earth.
Reconstruction of Voyager 2 images showing the Great Black spot (top left), Scooter (middle), and the Small Black Spot (lower right). Credit: NASA/JPL
Just like Jupiter and Saturn, Neptune has bands of storms that circle the planet. Astronomers have clocked winds on Neptune traveling at 2,100 km/hour, which is believed to be due to Neptune’s cold temperatures – which may decrease the friction in the system, During its 1989 flyby, NASA’s Voyager 2 spacecraft discovered the Great Dark Spot on Neptune.
Similar to Jupiter’s Great Red Spot, this is an anti-cyclonic storm measuring 13,000 km x 6,600 km across. A few years later, however, the Hubble Space Telescope failed to see the Great Dark Spot, but it did see different storms. This might mean that storms on Neptune don’t last as long as they do on Jupiter or even Saturn.
The more active weather on Neptune might be due, in part, to its higher internal heat. Although Neptune is much more distant than Uranus from the Sun, receiving 40% less sunlight, temperatures on the surface of the two planets are roughly similar. In fact, Neptune radiates 2.61 times as much energy as it receives from the Sun. This is enough heat to help drive the fastest winds in the Solar System.
Neptune is the second planet discovered in modern times. It was discovered at the same time by both Urbain Le Verrier and John Couch Adams. Neptune has only ever been visited by one spacecraft, Voyager 2, which made a fly by in August, 1989. Neptune has 13 known moons. Th largest and most famous of these is Triton, which is believed to be a former KBO that was captured by Neptune’s gravity.
Global Color Mosaic of Triton, taken by Voyager 2 in 1989. Credit: NASA/JPL/USGS
So much like Uranus, Neptune has a ring system, some intense weather patterns, and an impressive array of moons. Also like Uranus, Neptune’s composition allows for its aquamarine color; except that in Neptune’s case, this color is more intense and vivid. In addition, Neptune experiences some temperature anomalies that are yet to be explained. And let’s not forgt the raining diamonds!
And those are the planets in the Solar System thank you for joining the tour! Unfortunately, Pluto isn’t a planet any more, hence why it was not listed. We know, we know, take it up with the IAU!
Welcome, come in to the 468th and 469th Carnival of Space – we combined these two since it’s summer break for a lot of folks! The Carnival is a community of space science and astronomy writers and bloggers, who submit their best work each week for your benefit. I’m Susie Murph, part of the team at Universe Today and CosmoQuest. So now, on to this week’s stories! Continue reading “Carnival of Space #468-469”
New Horizons trajectory and the orbits of Pluto and 2014 MU69.
New Horizons trajectory and the orbits of Pluto and 2014 MU69.
In an ‘Independence Day’ gift to a slew of US planetary research scientists, NASA has granted approval to nine ongoing missions to continue for another two years this holiday weekend.
The biggest news is that NASA green lighted a mission extension for the New Horizons probe to fly deeper into the Kuiper Belt and decided to keep the Dawn probe at Ceres forever, rather than dispatching it to a record breaking third main belt asteroid.
And the exciting extension news comes just as the agency’s Juno probe is about to ignite a do or die July 4 fireworks display to achieve orbit at Jupiter – detailed here.
“Mission approved!” the researchers gleefully reported on the probes Facebook and Twitter social media pages.
“Our extended mission into the #KuiperBelt has been approved. Thanks to everyone for following along & hopefully the best is yet to come.
Dwarf planet Ceres is shown in this false-color renderings, which highlight differences in surface materials. The image is centered on Ceres brightest spots at Occator crater. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
The New Horizons spacecraft will now continue on course in the Kuiper Belt towards an small object known as 2014 MU69, to carry out the most distant close encounter with a celestial object in human history.
“Here’s to continued success!”
The spacecraft will rendezvous with the ancient rock on New Year’s Day 2019.
Researchers say that 2014 MU69 is considered as one of the early building blocks of the solar system and as such will be invaluable to scientists studying the origin of our solar system how it evolved.
It was almost exactly one year ago on July 14, 2015 that New Horizons conducted Earth’s first ever up close flyby and science reconnaissance of Pluto – the most distant planet in our solar system and the last of the nine planets to be explored.
Pluto Explored at Last. The New Horizons mission team celebrates successful flyby of Pluto in the moments after closest approach at 7:49 a.m. EDT on July 14, 2015. New Horizons Principal Investigator Alan Stern of Southwest Research Institute (SwRI), Boulder, CO., left, Johns Hopkins University Applied Physics Laboratory (APL) Director Ralph Semmel, center, and New Horizons Co-Investigator Will Grundy Lowell Observatory hold an enlarged print of an U.S. stamp with their suggested update after Pluto became the final planet in our solar system to be explored by an American space probe (crossing out the words ‘not yet’) – at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland. Credit: Ken Kremer/kenkremer.com
The immense volume of data gathered continues to stream back to Earth every day.
“The New Horizons mission to Pluto exceeded our expectations and even today the data from the spacecraft continue to surprise,” said NASA’s Director of Planetary Science Jim Green at NASA HQ in Washington, D.C.
“We’re excited to continue onward into the dark depths of the outer solar system to a science target that wasn’t even discovered when the spacecraft launched.”
This new global mosaic view of Pluto was created from the latest high-resolution images to be downlinked from NASA’s New Horizons spacecraft and released on Sept. 11, 2015. The images were taken as New Horizons flew past Pluto on July 14, 2015, from a distance of 50,000 miles (80,000 kilometers). This new mosaic was stitched from over two dozen raw images captured by the LORRI imager and colorized. Annotated with informal place names. Credits: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Marco Di Lorenzo/Ken Kremer/kenkremer.com
While waiting for news on whether NASA would approve an extended mission, the New Horizons engineering and science team already ignited the main engine four times to carry out four course changes in October and November 2015, in order to preserve the option of the flyby past 2014 MU69 on Jan 1, 2019.
Green noted that mission extensions into fiscal years 2017 and 2018 are not final until Congress actually passes sufficient appropriation to fund NASA’s Planetary Science Division.
“Final decisions on mission extensions are contingent on the outcome of the annual budget process.”
Tough choices were made even tougher because the Obama Administration has cut funding for the Planetary Sciences Division – some of which was restored by a bipartisan majority in Congress for what many consider NASA’s ‘crown jewels.’
NASA’s Dawn asteroid orbiter just completed its primary mission at dwarf planet Ceres on June 30, just in time for the global celebration known as Asteroid Day.
“The mission exceeded all expectations originally set for its exploration of protoplanet Vesta and dwarf planet Ceres,” said NASA officials.
The Dawn science team had recently submitted a proposal to break out of orbit around the middle of this month in order to this conduct a flyby of the main belt asteroid Adeona.
Green declined to approve the Dawn proposal, citing additional valuable science to be gathered at Ceres.
The long-term monitoring of Ceres, particularly as it gets closer to perihelion – the part of its orbit with the shortest distance to the sun — has the potential to provide more significant science discoveries than a flyby of Adeona,” he said.
The funding required for a multi-year mission to Adeona would be difficult in these cost constrained times.
However the spacecraft is in excellent shape and the trio of science instruments are in excellent health.
Dawn arrived at Ceres on March 6, 2015 and has been conducting unprecedented investigation ever since.
Dawn is Earth’s first probe in human history to explore any dwarf planet, the first to explore Ceres up close and the first to orbit two celestial bodies.
The asteroid Vesta was Dawn’s first orbital target where it conducted extensive observations of the bizarre world for over a year in 2011 and 2012.
The mission is expected to last until at least later into 2016, and possibly longer, depending upon fuel reserves.
Due to expert engineering and handling by the Dawn mission team, the probe unexpectedly has hydrazine maneuvering fuel leftover.
Dawn will remain at its current altitude at the Low Altitude Mapping Orbit (LAMO) for the rest of its mission, and indefinitely afterward, even when no further communications are possible.
Green based his decision on the mission extensions on the biannual peer review scientific assessment by the Senior Review Panel.
Dawn was launched in September 2007.
The other mission extensions – contingent on available resources – are: the Mars Reconnaissance Orbiter (MRO), Mars Atmosphere and Volatile EvolutioN (MAVEN), the Opportunity and Curiosity Mars rovers, the Mars Odyssey orbiter, the Lunar Reconnaissance Orbiter (LRO), and NASA’s support for the European Space Agency’s Mars Express mission.
Stay tuned here for Ken’s continuing Earth and planetary science and human spaceflight news.
We’ve had an abundance of news stories for the past few months, and not enough time to get to them all. So we are now using a tool called Trello to submit and vote on stories we would like to see covered each week, and then Fraser will be selecting the stories from there. Here is the link to the Trello WSH page (http://bit.ly/WSHVote), which you can see without logging in. If you’d like to vote, just create a login and help us decide what to cover!
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This mosaic of Pluto's surface was created from images taken by the New Horizons probe just 23 minutes before its closest approach. Credit: NASA/JHUAPL/SwRI
The New Horizons mission, which its conducted its historic flyby on July 14th, 2015, has yielded a wealth of scientific data about Pluto. This has included discoveries about Pluto’s size, its mountainous regions, its floating ice hills, and (more recently) how the dwarf planet interacts with solar wind – a discovery which showed that Pluto is actually more planet-like than previously thought.
But beyond revelations about the planet’s size, geography and surface features, it has also provided the most breathtaking, clear, and inspiring images of Pluto and its moons to date. And with this latest release of images taken by the New Horizon‘s Long Range Reconnaissance Imager (LORRI), people here on Earth are being treated to be the best close-up of Pluto yet.
These images, which were taken while the New Horizon’s probe was still 15,850 km (9,850 mi) away from Pluto (just 23 minutes before it made its closest approach), extend across the hemisphere that the probe was facing as it flew past. It shows features ranging from the cratered northern uplands and the mountainous regions in Voyager Terra before slicing through the flatlands of “Pluto’s Heart” – aka. Tombaugh Regio – and ending up in another stretch of rugged highlands.
Informal names given to Pluto’s surface features. Credit: earthsky.org
The width of the strip varies as the images pass from north to south, from more than 90 km (55 mi) across at the northern end to about 75 km (45 mi) at its southern point. The perspective also changes, with the view appearing virtually horizontal at the northern end and then shifting to an almost top-down view onto the surface by the end.
The crystal clear photographs that make up the mosaic – which have a resolution of about 80 meters (260 feet) per pixel – offer the most detailed view of Pluto’s surface ever. With this kind of clarity, NASA scientists are able to discern features that were never before visible, and learn things about the kinds of geological processes which formed them.
This includes the chaotic nature of the mountains in the northern hemisphere, and the varied nature of the icy nitrogen plains across Tombaugh Regio – which go from being cellular, to non-cellular, to a cross-bedding pattern. These features are a further indication that Pluto’s surface is the product of a combination of geological forces, such as cryovolcanism, sublimation, geological activity, convection between water and nitrogen ice, and interaction between the surface and atmosphere.
Images snapped by New Horizons’ Long Range Reconnaissance Imager (LORRI) while the probe was still on approach to Pluto were combined with color data from the Ralph instrument to create this global view of Pluto. Credits: NASA/JHUAPL/SwRI
Alan Stern, the principal investigator of the New Horizons mission and the Associate Vice President of Research and Development at the Southwest Research Institute, was especially impressed with this latest find. As he told Universe Today via email:
“This new high resolution image mosaic is the complete highest resolution strip of images New Horizons obtained, and its both eye candy gorgeous and scientifically rich. Think about it— one flyby and we have this mosaic, plus so much more; no dataset like this existed on Mars until we’d flown half a dozen missions there!”
The most distant flyby in the history of space exploration, and yet we’ve obtained more from this one mission than multiple flybys were able to provide from one of Earth’s closest neighbors. Fascinating! And what’s more, new information is expected to be coming from the New Horizons probe until this coming October. To top it off, our scientists are still not finished analyzing all the information the mission collected during its flyby.
The full-resolution image can be viewed here, and be sure to enjoy this NASA video of the mosaic:
Neptune photographed by Voyager 2. Image credit: NASA/JPL
The Solar System is pretty huge place, extending from our Sun at the center all the way out to the Kuiper Cliff – a boundary within the Kuiper Belt that is located 50 AU from the Sun. As a rule, the farther one ventures from the Sun, the colder and more mysterious things get. Whereas temperatures in the inner Solar System are enough to burn you alive or melt lead, beyond the “Frost Line“, they get cold enough to freeze volatiles like ammonia and methane.
So what is the coldest planet of our Solar System? In the past, the title for “most frigid body” went to Pluto, as it was the farthest then-designated planet from the Sun. However, due to the IAU’s decision in 2006 to reclassify Pluto as a “dwarf planet”, the title has since passed to Neptune. As the eight planet from our Sun, it is now the outermost planet in the Solar System, and hence the coldest.
Orbit and Distance:
With an average distance (semi-major axis) of 4,504,450,000 km (2,798,935,466.87 mi or 30.11 AU), Neptune is the farthest planet from the Sun. The planet has a very minor eccentricity of 0.0086, which means that its orbit around the Sun varies from a distance of 29.81 AU (4.459 x 109 km) at perihelion to 30.33 AU (4.537 x 109 km) at aphelion.
The Solar System. Credit: NASA
Because Neptune’s axial tilt (28.32°) is similar to that of Earth (~23°) and Mars (~25°), the planet experiences similar seasonal changes. Combined with its long orbital period, this means that the seasons last for forty Earth years. Also owing to its axial tilt being comparable to Earth’s is the fact that the variation in the length of its day over the course of the year is not any more extreme than it is on Earth.
Average Temperature:
When it comes to ascertaining the average temperature of a planet, scientists rely on temperature variations measured from the surface. As a gas/ice giant, Neptune has no surface, per se. As a result, scientists rely on temperature readings from where the atmospheric pressure is equal to 1 bar (100 kPa), the equivalent to atmospheric pressure at sea level here on Earth.
On Neptune, this area of the atmosphere is just below the upper level clouds. Pressures in this region range between 1 and 5 bars (100 – 500 kPa), and temperature reach a high of 72 K (-201.15 °C; -330 °F). At this temperature, conditions are suitable for methane to condense, and clouds of ammonia and hydrogen sulfide are thought to form (which is what gives Neptune its characteristically dark cyan coloring).
Farther into space, where pressures drop to about 0.1 bars (10 kPa), temperatures decrease to their low of around 55 K (-218 °C; -360 °F). Further into the planet, pressures increase dramatically, which also leads to a dramatic increase in temperature. At its core, Neptune reaches temperatures of up to 7273 K (7000 °C; 12632 °F), which is comparable to the surface of the Sun.
Neptune Great Dark Spot in High Resolution. Credit: NASA/JPL
The huge temperature differences between Neptune’s center and its surface (along with its differential rotation) create huge wind storms, which can reach as high as 2,100 km/hour, making them the fastest in the Solar System. The first to be spotted was a massive anticyclonic storm measuring 13,000 x 6,600 km and resembling the Great Red Spot of Jupiter.
Known as the Great Dark Spot, this storm was not spotted five later (Nov. 2nd, 1994) when the Hubble Space Telescope looked for it. Instead, a new storm that was very similar in appearance was found in the planet’s northern hemisphere, suggesting that these storms have a shorter lifespan than Jupiter’s. The Scooter is another storm, a white cloud group located farther south than the Great Dark Spot.
This nickname first arose during the months leading up to the Voyager 2 encounter in 1989, when the cloud group was observed moving at speeds faster than the Great Dark Spot. The Small Dark Spot, a southern cyclonic storm, was the second-most-intense storm observed during the 1989 encounter. It was initially completely dark; but as Voyager 2 approached the planet, a bright core developed and could be seen in most of the highest-resolution images.
Temperature Anomalies:
Despite being 50% further from the Sun than Uranus – which orbits the Sun at an average distance of 2,875,040,000 km (1,786,467,032.5 mi or 19.2184 AU) – Neptune receives only 40% of the solar radiation that Uranus does. In spite of that, the two planets’ surface temperatures are surprisingly close, with Uranus experiencing an average “surface” temperature of 76 K (-197.2 °C)
Four images of Neptune taken a few hours apart by the Hubble Space Telescope on June 25-26, 2011. Credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA)
And while temperatures similarly increase the further one ventures into the core, the discrepancy is larger. Uranus only radiates 1.1 times as much energy as it receives from the Sun, whereas Neptune radiates about 2.61 times as much. Neptune is the farthest planet from the Sun, yet its internal energy is sufficient to drive the fastest planetary winds seen in the Solar System.
One would expect Neptune to be much colder than Uranus, and the mechanism for this remains unknown. However, astronomers have theorized that Neptune’s higher internal temperature (and the exchange of heat between the core and outer layers) might be the reason for why Neptune isn’t significantly colder than Uranus.
As already noted, Pluto’s surface temperatures do get to being lower than Neptune’s. Between its greater distance from the Sun, and the fact that it is not a gas/ice giant (so therefore doesn’t have extreme temperatures at its core) means that it experiences temperatures between a high of 55 K (-218 °C; -360 °F)and a low of 33 K (-240 °C; -400 °F). However, since it is no longer classified as a planet (but a dwarf planet, TNO, KBO, plutoid, etc.) it is no longer in the running. Sorry, Pluto!
New Horizon's July 2015 flyby of Pluto captured this iconic image of the heart-shaped region called Tombaugh Regio. Credit:
NASA/JHUAPL/SwRI.
Pluto can’t seem to catch a break lately. After being reclassified in 2006 by the International Astronomical Union, it seemed that what had been the 9th planet of the Solar System was now relegated to the status of “dwarf planet” with the likes of Ceres, Eris, Haumea, and Makemake. Then came the recent announcements that the title of “Planet 9” may belong to an object ten times the mass of Earth located 700 AU from our Sun.
And now, new research has been produced that indicates that Pluto may need to be reclassified again. Using data provided by the New Horizons mission, researchers have shown that Pluto’s interaction with the Sun’s solar wind is unlike anything observed in the Solar System thus far. As a result, it would seem that the debate over how to classify Pluto, and indeed all astronomical bodies, is not yet over.
Artist's impression of the Earth scorched by our Sun as it enters its Red Giant Branch phase. Credit: Wikimedia Commons/Fsgregs
Since the beginning of human history, people have understood that the Sun is a central part of life as we know it. It’s importance to countless mythological and cosmological systems across the globe is a testament to this. But as our understand of it matured, we came to learn that the Sun was here long before us, and will be here long after we’re gone. Having formed roughly 4.6 bullion years ago, our Sun began its life roughly 40 million years before our Earth had formed.
Since then, the Sun has been in what is known as its Main Sequence, where nuclear fusion in its core causes it to emit energy and light, keeping us here on Earth nourished. This will last for another 4.5 – 5.5 billion years, at which point it will deplete its supply of hydrogen and helium and go through some serious changes. Assuming humanity is still alive and calls Earth home at this time, we may want to consider getting out the way!
The Birth of Our Sun:
The predominant theory on how our Sun and Solar System formed is known as Nebular Theory, which states that the Sun and all the planets began billions of years ago as a giant cloud of molecular gas and dust. Then, approximately 4.57 billion years ago, this cloud experienced gravitational collapse at its center, where anything from a passing star to a shock wave caused by a supernova triggered the process that led to our Sun’s birth.
Basically, this took place after pockets of dust and gas began to collect into denser regions. As these regions pulled in more and more matter, conservation of momentum caused them to begin rotating, while increasing pressure caused them to heat up. Most of the material ended up in a ball at the center while the rest of the matter was flattened out into a large disk that circled around it.
Young stars have a disk of gas and dust around them called a protoplanetary disk. Out of this disk planets are formed, and the presence of water ice in the disc affects where different types of planets form. Credit: NASA/JPL-Caltech
The ball at the center would eventually form the Sun, while the disk of material would form the planets. The Sun then spent the next 100,000 years as a collapsing protostar before temperature and pressures in the interior ignited fusion at its core. The Sun started as a T Tauri star – a wildly active star that blasted out an intense solar wind. And just a few million years later, it settled down into its current form.
Main Sequence:
For the past 4.57 billion years (give or take a day or two), the Sun has been in the Main Sequence of its life. This is characterized by the process where hydrogen fuel, under tremendous pressure and temperatures in its core, is converted into helium. In addition to changing the properties of its constituent matter, this process also produces a tremendous amount of energy. All told, every second, 600 million tons of matter are converted into neutrinos, solar radiation, and roughly 4 x 1027 Watts of energy.
Naturally, this process cannot last forever since it is dependent on the presence of matter which is being regularly consumed. As time goes on and more hydrogen is converted into helium, the core will continue to shrink, allowing the outer layers of the Sun to move closer to the center and experience a stronger gravitational force.
This will place more pressure on the core, which is resisted by a resulting increase in the rate at which fusion occurs. Basically, this means that as the Sun continues to expend hydrogen in its core, the fusion process speeds up and the output of the Sun increases. At present, this is leading to a 1% increase in luminosity every 100 million years, and a 30% increase over the course of the last 4.5 billion years.
The life cycle of a Sun-like star, from its birth on the left side of the frame to its evolution into a red giant on the right after billions of years. Credit: ESO/M. Kornmesser
Approximately 1.1 billion years from now, the Sun will be 10% brighter than it is today. This increase in luminosity will also mean an increase in heat energy, one which the Earth’s atmosphere will absorb. This will trigger a runaway greenhouse effect that is similar to what turned Venus into the terrible hothouse it is today.
In 3.5 billion years, the Sun will be 40% brighter than it is right now, which will cause the oceans to boil, the ice caps to permanently melt, and all water vapor in the atmosphere to be lost to space. Under these conditions, life as we know it will be unable to survive anywhere on the surface, and planet Earth will be fully transformed into another hot, dry world, just like Venus.
Red Giant Phase:
In 5.4 billion years from now, the Sun will enter what is known as the Red Giant phase of its evolution. This will begin once all hydrogen is exhausted in the core and the inert helium ash that has built up there becomes unstable and collapses under its own weight. This will cause the core to heat up and get denser, causing the Sun to grow in size.
It is calculated that the expanding Sun will grow large enough to encompass the orbit’s of Mercury, Venus, and maybe even Earth. Even if the Earth were to survive being consumed, its new proximity to the the intense heat of this red sun would scorch our planet and make it completely impossible for life to survive. However, astronomers have noted that as the Sun expands, the orbit of the planet’s is likely to change as well.
When the Sun reaches this late stage in its stellar evolution, it will lose a tremendous amount of mass through powerful stellar winds. Basically, as it grows, it loses mass, causing the planets to spiral outwards. So the question is, will the expanding Sun overtake the planets spiraling outwards, or will Earth (and maybe even Venus) escape its grasp?
K.-P Schroder and Robert Cannon Smith are two researchers who have addressed this very question. In a research paper entitled “Distant Future of the Sun and Earth Revisted” which appeared in the Monthly Notices of the Royal Astronomical Society, they ran the calculations with the most current models of stellar evolution.
According to Schroder and Smith, when the Sun becomes a red giant star in 7.59 billion years, it will start to lose mass quickly. By the time it reaches its largest radius, 256 times its current size, it will be down to only 67% of its current mass. When the Sun does begin to expand, it will do so quickly, sweeping through the inner Solar System in just 5 million years.
It will then enter its relatively brief (130 million year) helium-burning phase, at which point, it will expand past the orbit of Mercury, and then Venus. By the time it approaches the Earth, it will be losing 4.9 x 1020 tonnes of mass every year (8% the mass of the Earth).
But Will Earth Survive?:
Now this is where things become a bit of a “good news/bad news” situation. The bad news, according to Schroder and Smith, is that the Earth will NOT survive the Sun’s expansion. Even though the Earth could expand to an orbit 50% more distant than where it is today (1.5 AUs), it won’t get the chance. The expanding Sun will engulf the Earth just before it reaches the tip of the red giant phase, and the Sun would still have another 0.25 AU and 500,000 years to grow.
Artist’s impression of a Red giant star. Credit:NASA/ Walt Feimer
Once inside the Sun’s atmosphere, the Earth will collide with particles of gas. Its orbit will decay, and it will spiral inward. If the Earth were just a little further from the Sun right now, at 1.15 AU, it would be able to survive the expansion phase. If we could push our planet out to this distance, we’d also be in business. However, such talk is entirely speculative and in the realm of science fiction at the moment.
And now for the good news. Long before our Sun enters it’s Red Giant phase, its habitable zone (as we know it) will be gone. Astronomers estimate that this zone will expand past the Earth’s orbit in about a billion years. The heating Sun will evaporate the Earth’s oceans away, and then solar radiation will blast away the hydrogen from the water. The Earth will never have oceans again, and it will eventually become molten.
Yeah, that’s the good news… sort of. But the upside to this is that we can say with confidence that humanity will be compelled to leave the nest long before it is engulfed by the Sun. And given the fact that we are dealing with timelines that are far beyond anything we can truly deal with, we can’t even be sure that some other cataclysmic event won’t claim us sooner, or that we wont have moved far past our current evolutionary phase.
An interesting side benefit will be how the changing boundaries of our Sun’s habitable zone will change the Solar System as well. While Earth, at a mere 1.5 AUs, will no longer be within the Sun’s habitable zone, much of the outer Solar System will be. This new habitable zone will stretch from 49.4 AU to 71.4 AU – well into the Kuiper Belt – which means the formerly icy worlds will melt, and liquid water will be present beyond the orbit of Pluto.
Perhaps Eris will be our new homeworld, the dwarf planet of Pluto will be the new Venus, and Haumeau, Makemake, and the rest will be the outer “Solar System”. But what is perhaps most fascinating about all of this is how humans are even tempted to ask “will it still be here in the future” in the first place, especially when that future is billions of years from now.
Somehow, the subjects of what came before us, and what will be here when we’re gone, continue to fascinate us. And when dealing with things like our Sun, the Earth, and the known Universe, it becomes downright necessary. Our existence thus far has been a flash in the pan compared to the cosmos, and how long we will endure remains an open question.
A view of Ceres in natural colour, pictured by the Dawn spacecraft in May 2015. Credit: NASA/ JPL/Planetary Society/Justin Cowart
We continue our “Definitive Guide to Terraforming” series with a look at another body in our Solar System – the dwarf planet Ceres. Like many moons in the outer Solar System, Ceres is a world of ice and rock, and is the largest body in the Asteroid Belt. Humans beings could one day call it home, but could its surface also be made “Earth-like”?
In the Solar System’s Main Asteroid Belt, there are literally millions of celestial bodies to be found. And while the majority of these range in size from tiny rocks to planetesimals, there are also a handful of bodies that contain a significant percentage of the mass of the entire Asteroid Belt. Of these, the dwarf planet Ceres is the largest, constituting of about a third of the mass of the belt and being the sixth-largest body in the inner Solar System by mass and volume.
In addition to its size, Ceres is the only body in the Asteroid Belt that has achieved hydrostatic equilibrium – a state where an object becomes rounded by the force of its own gravity. On top of all that, it is believed that this dwarf planet has an interior ocean, one which contains about one-tenth of all the water found in the Earth’s oceans. For this reason, the idea of colonizing Ceres someday has some appeal, as well as terraforming.
