More “Hollowed Ground” on Mercury

MESSENGER captures image of curious "hollows" around a crater peak


The latest featured image from NASA’s MESSENGER spacecraft, soon to complete its first year in orbit around Mercury, shows the central peak of the 78-mile (138-km) – wide crater Eminescu surrounded by more of those brightly-colored surface features dubbed “hollows”. Actually tinted a light blue color, hollows may be signs of an erosion process unique to Mercury because of its composition and close proximity to the Sun.

First noted in September of last year, hollows have now been identified in many areas across Mercury. They showed up in previous images as only bright spots, but once MESSENGER established orbit in March of 2011 and began high-resolution imaging of Mercury’s surface it became clear that these features were something totally new.

The lack of craters within hollows seems to indicate that they are relatively young features. In fact, they may be part of a process that continues even now.

“Analysis of the images and estimates of the rate at which the hollows may be growing led to the conclusion that they could be actively forming today,” said David Blewett of the Johns Hopkins University Applied Physics Laboratory (APL).

One hypothesis is that the hollows are formed by the sublimation of subsurface material exposed during the creation of craters, around which they are most commonly seen. Being so close to the Sun (29 million miles/46 million km at closest) and lacking a protective atmosphere like Earth’s, Mercury is constantly being scoured by the powerful solar wind. This relentless stream of charged particles may literally be “sandblasting” exposed volatile materials off the planet’s surface!

The image above shows an area approximately 41 miles (66 km) across. It has been rotated to enhance perspective; see the original image and caption here.

Image: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

NASA’s Pluto Probe Marks a New Milestone

Artist's impression of New Horizons' encounter with Pluto and Charon. Credit: NASA/Thierry Lombry


It may not have noticed anything different as it continued its high-speed trek through interplanetary space, but today New Horizons passed a new milestone: it is now (and will be for quite some time) the closest spacecraft ever to Pluto!

This breaks the previous record held by Voyager 1, which came within 983 million miles (1.58 billion km) of the dwarf planet on January 29, 1986.

New Horizons has been traveling through the solar system since its launch on January 19, 2006 and is now speeding toward Pluto at around 34,500 mph (55,500 km/hr). It has thus far traveled for 2,143 days and is just over halfway to the distant icy world.

“Although we’re still a long way — 1.5 billion kilometers from Pluto — we’re now in new territory as the closest any spacecraft has ever gotten to Pluto, and getting closer every day by over a million kilometers.”

– Alan Stern, New Horizons Principal Investigator

A gravity boost obtained by a close pass of Jupiter in 2007 gave the spacecraft the extra speed needed to make it to Pluto by 2015. (Without that, it wouldn’t have been reaching Pluto until 2036!)

Achievements like this are wonderful indicators that New Horizons is alive and well and that its historic goal is getting increasingly closer every day.

Diagram of the Pluto-Charon encounter in July 2015 (NASA/APL)

“We’ve come a long way across the solar system,” said Glen Fountain, New Horizons project manager at the Johns Hopkins University Applied Physics Laboratory (APL). “When we launched it seemed like our 10-year journey would take forever, but those years have been passing us quickly. We’re almost six years in flight, and it’s just about three years until our encounter begins.”

See answers to some FAQs about Pluto

New Horizons will pass by Pluto and its moons on July 14, 2015, becoming the first spacecraft ever to visit the distant system. It will image Pluto’s surface in unprecedented detail, resolving features as small as 200 feet (60 meters) across.

New Horizons will not land or enter orbit around Pluto but instead quickly pass by and continue on into the Kuiper Belt, where even more distant frozen worlds await. The New Horizons team is currently investigating further exploration targets should its mission be extended.

 Read more on the New Horizons mission site.

The New Horizons mission timeline (click to enlarge). Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.


New Research Finds Venus’ Winds, They Are A-Changin’

Image of Venus in ultraviolet light by ESA's Venus Express.



Venus, Earth’s hotheaded neighbor, may have more variability in its weather patterns than previously believed. Using infrared data obtained by ground-based telescopes in Hawaii and Arizona researchers have found that Venus’ mesosphere and thermosphere are less consistent in temperature than layers closer to its surface.

But first let’s talk about Venus itself.

Possibly the most inhospitable of planets in our solar system, Venus is the victim of a runaway greenhouse effect. Our neighboring world is a virtual oven… with a rocky surface baked by 800ºF temperatures and crushed beneath the weight of its own incredibly dense atmosphere, standing “sea level” on Venus would be like being 3,300 feet underwater, just in terms of pressure per square inch. And as if the heat and pressure weren’t enough, Venus’ skies are full of clouds made of corrosive sulphuric acid, lit by bolts of lightning and and whipped along by hurricane-force planetwide winds. All Earth-based probes that have ever landed there only lasted moments on the surface before succumbing to Venus’ destructive environment.

Venus is, quite literally, hellish.

Venus' south polar vortex imaged in infrared. A darker region corresponds to higher temperature and thus lower altitude. Credit: ESA/VIRTIS/INAF-IASF/Obs. de Paris-LESIA.

Unlike Earth, Venus does not have much of an axial tilt. This means there’s little, if any, seasonal variation on Venus. (Actually it does have a tilt… Venus is rotated almost completely upside-down relative to its poles, and so in effect still has very little axial tilt.) And since its cloud cover is so dense and it lacks a hydrologic cycle to move heat energy around, it pretty much stays at a constant level of “extreme broil” all across Venus’ surface.

Surface weather on Venus, although unpleasant, is consistent.

Yet based on an international team’s new research this is not the case higher up in Venus’ atmosphere. A new look at old data has uncovered changing weather patterns visible in infrared light at about 68 miles (110 kilometers) above the planet’s surface in the cold, clear air above the acid clouds.

“Any variability in the weather on Venus is noteworthy, because the planet has so many features to keep atmospheric conditions the same,” said Dr. Tim Livengood, a researcher with the National Center for Earth and Space Science Education and the University of Maryland, now stationed at NASA’s Goddard Space Flight Center in Greenbelt.

Dr. Theodor Kostiuk of NASA Goddard explains further: “Although the air over the polar regions in these upper atmospheric layers on Venus was colder than the air over the equator in most measurements, occasionally it appeared to be warmer. In Earth’s atmosphere, a circulation pattern called a ‘Hadley cell’ occurs when warm air rises over the equator and flows toward the poles, where it cools and sinks. Since the atmosphere is denser closer to the surface, the descending air gets compressed and warms the upper atmosphere over Earth’s poles. We saw the opposite on Venus.”

Many factors could be contributing to Venus’ upper-atmospheric variabilities, such as interactions between opposing winds blowing around the planet at over 200 mph, giant vortexes that churn around its poles, and possibly even solar activity, like solar storms and coronal mass ejections which may create turbulence in Venus’ upper atmosphere.

“The mesosphere and thermosphere of Venus are dynamically active. Wind patterns resulting from solar heating and east to west zonal winds compete, possibly resulting in altered local temperatures and their variability over time.”

– Lead author Dr. Guido Sonnabend, University of Cologne, Germany

Artist concept of Venus' surface. (NASA)

The team also found that the temperatures of Venus’ atmosphere change over time, spanning weeks, months, years… even decades. Temperatures measured in 1990-91 are warmer than in 2009, and equatorial temperatures were even warmer in 2007.

“In addition to all these changes, we saw warmer temperatures than those predicted for this altitude by the leading accepted model,” said Kostiuk. “This tells us that we have lots of work to do updating our upper atmospheric circulation model for Venus.”

Even though Venus is compositionally similar to Earth and has a similar size as well, at some point in its history it lost all of its water to space and became the cloud-covered oven it is today. Studying Venus will help scientists learn how this may have happened and – hopefully! –  learn how to prevent the same fate from ever befalling Earth.

The paper, led by Dr. Guido Sonnabend of the University of Cologne, Germany and co-authored by Drs. Livengood and Kostiuk, appeared July 23 in the online edition of the journal Icarus.

Read more on the NASA feature article here.

Mars May Have Once Been a Cold, Wet World

2 billion years ago Mars may have featured a frigid ocean. Credit: Taylor Perron/UC Berkeley.


Many planetary scientists suspect that Mars, now cold and very dry, once had a liquid water ocean covering parts of its surface. But this does not necessarily mean that the Red Planet was ever a tropical paradise… a recent paper by a team of astrobiologists suggests that Mars was much more bitter than balmy.

Astrobiologist Alberto Fairyn and colleagues have published a paper in the journal Nature Geoscience suggesting that the marked absence of phyllosilicates in Mars’ northern lowlands is indicative of a cold ocean environment, with perhaps even a boundary of frozen glaciers.

Phyllosilicates are minerals that, on Earth, are found readily in marine sediments and sedimentary rock that was formed in the presence of an ocean environment. These same minerals have also been seen via orbiting spacecraft spectrometers to be present in sediments located in Mars’ equatorial regions, but not in the northern latitudes. Fairyn and his team, intrigued by the disparity between existing models that described Mars as being once warm and wet and the lack of phyllosilicates in the north, used new climatic and geochemical models to deduce that Mars’ northern oceans must have been consistently near freezing, with portions even covered over by ice.

Did Mars once have ice-covered seas? (Original image © Maggie & David. Edited by J. Major.)

The current presence of moraines in the northern highlands also suggests that glaciers may have surrounded these frigid seas, which may have prevented the transportation of phyllosilicates down to the northern ocean basin. Again, to use our own planet as an analogy, moraines are rocky debris left over from the movement of glaciers. Their existence on Mars strongly suggests a period of early glaciation.

The research by Fairyn et al. contradict – or, more aptly, combine –  two leading concepts of early Mars: one, that it was cold and dry and the existence of any liquid water was restricted to the equator for small periods of time; and two, that it was once globally warmer and wetter and sustained rivers, lakes and oceans of liquid water for extended periods.

Thus a cold Mars with an Arctic, icy ocean seems to be a more fitting causation of the current state of the planet, suggests Fairyn.

More research is planned, including running through more low-temperature models and hunting for ancient coastal areas that may have been impacted by icebergs. This will no doubt prove to be a challenge since much of the evidence is now buried deep beneath newer sediments and volcanic deposits. Still, Fairyn is confident that his model may help solve a long-standing debate over the history of the Red Planet.

Read more in an article by Bob Yirka on PhysOrg.


Jason Major is a graphic designer, photo enthusiast and space blogger. Visit his website Lights in the Dark and follow him on Twitter @JPMajor or on Facebook for the most up-to-date astronomy awesomeness!

More Surprises From Pluto

Artist's illustration of Pluto's surface. Credit: NASA


Ah, Pluto. Seems every time we think we’ve got it figured out, it has a new surprise to throw at us.

First spotted in 1930 by a young Clyde Tombaugh, for 76 years it enjoyed a comfortable position as the solar system’s most distant planet. Then a controversial decision in 2006 by the International Astronomical Union, spurred by suggestions from astronomer (and self-confessed “planet-killer”) Mike Brown*, relegated Pluto to a new class of worlds called “dwarf planets”. Not quite planets and not quite asteroids, dwarf planets cannot entirely clear their orbital path with their own gravitational force and thus miss out on full planetary status. Besides immediately making a lot of science textbooks obsolete and rendering the handy mnemonic “My Very Eager Mother Just Served Us Nine Pies” irrelevant (or at least confusing), the decision angered many people around the world, both in and out of the scientific community. Pluto is a planet, they said, it always has been and always will be! Save Pluto! the schoolkids wrote in crayon to planetarium directors. The world all of a sudden realized how much people liked having Pluto as the “last” planet, and didn’t want to see it demoted by decision, especially a highly contested one.

Yet as it turns out, Pluto really may not be a planet after all.

It may be a comet.

But…that’s getting ahead of ourselves. First things first.

Discovery data showing carbon monoxide spectrum. Credit: J.S. Greaves / Joint Astronomy Centre.

Recent discoveries by a UK team of astronomers points to the presence of carbon monoxide in Pluto’s atmosphere. Yes, Pluto has an atmosphere; astronomers have known about it since 1988. At first assumed to be about 100km thick, it was later estimated to extend out about 1500km and be composed of methane gas and nitrogen. This gas would expand from the planet’s – er, dwarf planet’s – surface as it came closer to the Sun during the course of its eccentric 248-year orbit and then freeze back onto the surface as it moved further away. The new findings from the University of St Andrews team, made by observations with the James Clerk Maxwell telescope in Hawaii, identify an even thicker atmosphere containing carbon monoxide that extends over 3000 km, reaching nearly halfway to Pluto’s largest moon, Charon.

It’s possible that this carbon monoxide atmosphere may have expanded outwards from Pluto, especially in the years since 1989 when it made the closest approach to the Sun in its orbit. Surface heating (and the term “heating” is used scientifically here…remember, at around -240ºC (-400ºF) Pluto would seem anything but balmy to us!) by the Sun’s radiation would have warmed the surface and expelled these gases outwards. This also coincides with observations made by the Hubble Space Telescope over the course of four years, which revealed varying patterns of dark and light areas on Pluto’s surface – possibly caused by the thawing of frozen areas that shift and reveal lighter surface material below.

“Seeing such an example of extra-terrestrial climate-change is fascinating. This cold simple atmosphere that is strongly driven by the heat from the Sun could give us important clues to how some of the basic physics works, and act as a contrasting test-bed to help us better understand the Earth’s atmosphere.”

–  Dr. Jane Greaves, Team Leader

In fact, carbon monoxide may be the key to why Pluto even still has an atmosphere. Unlike methane, which is a greenhouse gas, carbon monoxide acts as a coolant; it may be keeping Pluto’s fragile atmosphere from heating up too much and escaping into space entirely! Over the decades and centuries that it takes for Pluto to complete a single year, the balance between these two gases must be extremely precise.

Read more about this discovery on the Royal Astronomical Society’s site.

Pluto's elliptical orbit

So here we have Pluto exhibiting an expanding atmosphere of thawing expelled gas as it gets closer to the Sun in an elliptical, eccentric orbit. (Sound familiar?) And now there’s another unusual, un-planet-like feature that’s being put on the table: Pluto may have a tail.

Actually this is an elaboration of the research results coming from the same team at the University of St Andrews. The additional element here is a tiny redshift detected in the carbon monoxide signature, indicating that it is moving away from us in an unusual way. It’s possible that this could be caused by the top layers of Pluto’s atmosphere – where the carbon monoxide resides – being blown back by the solar wind into, literally, a tail.

That sounds an awful lot, to this particular astronomy reporter anyway, like a comet.

Just saying.

Anyway, regardless of what Pluto is or isn’t, will be called or used to be called, there’s no denying that it is a fascinating little world that deserves our attention. (And it will be getting plenty of that come July 2015 when the New Horizons spacecraft swings by for a visit!) I’m sure there’s no one here who would argue that fact.

New Horizons’ upcoming visit will surely answer many questions about Pluto – whatever it is – and most likely raise even more.


Artist's impression of Pluto's huge atmosphere of carbon monoxide.Credit:P.A.S. Cruickshank.

The new discovery was presented by team leader Dr. Jane Greaves on Wednesday, April 20 at the National Astronomy Meeting in Wales.

Article reference: Discovery Of Carbon Monoxide In The Upper Atmosphere Of Pluto


*No disrespect to Mr. Brown intended…he was just performing science as he saw fit!



Which Planet Has The Longest Day?

Mariner 10's Venus. Image: NASA
Mariner 10's Venus. Image: NASA

Just to be clear, this answer to ‘which planet has the longest day’ is based on this criteria: a planets day is how long it takes it to complete one rotation on its axis. This is also referred to as its rotational period. So, Venus has the longest day of any planet in our solar system. It completes one rotation every 243 Earth days. Its day lasts longer than its orbit. It orbits the Sun every 224.65 Earth days, so a day is nearly 20 Earth days longer than its year.

Length Of A Day On The Planets In Our Solar System

Mercury: 58 days and 15 hours
Venus: 243 days
Mars: 24 hours, 39 minutes and 35 seconds
Jupiter: 9.9 hours
Saturn: 10 hours 45 minutes 45 seconds, but can only be approximated because of atmospheric density.
Uranus: 17 hours, 14 minutes and 24 seconds
Neptune: 16 hours, 6 minutes and 36 seconds, but it is a bit more complicated than that. The equator and poles rotate at different speeds. You would have to do more research on the planet to fully understand the varying day on Neptune.

Now, back to why the Venusian day is longer than its year. Venus is closer to the Sun; therefore, its orbit takes a shorter period of time than its rotation upon its axis. The planet also rotates in retrograde. That means it spins in the opposite direction of the Earth. If you were standing on Venus, you could see the Sun rise in the West and set in the East.

A manned Venus flyby mission was proposed in the late 1960s. The mission was planned to launch in late October or early November 1973, and would have used a Saturn V rocket to send three men. The flight would have lasted approximately one year. The spacecraft would have passed approximately 5,000 km from the surface about four months into the flight. There have been several unmanned probes and flybys of the planet, including MESSENGER and the Venus Express. Future proposed missions include the BepiColombo, Venus InSitu Explorer, and the Venera-D.

We have written many articles about Venus for Universe Today. Here are some interesting facts about planet Venus, and here are some pictures of planet Venus.

If you’d like more information on Venus, check out Hubblesite’s News Releases about Venus, and here’s a link to NASA’s Solar System Exploration Guide on Venus.

We’ve also recorded an entire episode of Astronomy Cast all about Venus. Listen here, Episode 50: Venus.

Planets Fact Sheet

Mass: 0.3302 x 1024 kg
Volume: 6.083 x 1010 km3
Average radius: 2439.7 km
Average diameter: 4879.4 km
Mean density: 5.427 g/cm3
Escape velocity: 4.3 km/s
Surface gravity: 3.7 m/s2
Visual magnitude: -0.42
Natural satellites: 0
Rings? – No
Semimajor axis: 57,910,000 km
Orbit period: 87.969 days
Perihelion: 46,000,000 km
Aphelion: 69,820,000 km
Mean orbital velocity: 47.87 km/s
Maximum orbital velocity: 58.98 km/s
Minimum orbital velocity: 38.86 km/s
Orbit inclination: 7.00°
Orbit eccentricity: 0.2056
Sidereal rotation period: 1407.6 hours
Length of day: 4222.6 hours
Discovery: Known since prehistoric times
Minimum distance from Earth: 77,300,000 km
Maximum distance from Earth: 221,900,000 km
Maximum apparent diameter from Earth: 13 arc seconds
Minimum apparent diameter from Earth: 4.5 arc seconds
Maximum visual magnitude: -1.9

Mass: 4.8685 x 1024 kg
Volume: 92.843 x 1010 km3
Average radius: 6051.8 km
Average diameter: 12103.6 km
Mean density: 5.243 g/cm3
Escape velocity: 10.36 km/s
Surface gravity: 8.87 m/s2
Visual magnitude: -4.40
Natural satellites: 0
Rings? – No
Semimajor axis: 108,210,000 km
Orbit period: 224.701 days
Perihelion: 107,480,000 km
Aphelion: 108,940,000 km
Mean orbital velocity: 35.02 km/s
Maximum orbital velocity: 35.26 km/s
Minimum orbital velocity: 34.79 km/s
Orbit inclination: 3.39°
Orbit eccentricity: 0.0067
Sidereal rotation period: 5832.5 hours
Length of day: 2802.0 hours
Discovery: Known since prehistoric times
Minimum distance from Earth: 38,200,000 km
Maximum distance from Earth: 261,000,000 km
Maximum apparent diameter from Earth: 66.0 arc seconds
Minimum apparent diameter from Earth: 9.7 arc seconds
Maximum visual magnitude: -4.6

Mass: 5.9736 x 1024 kg
Volume: 108.321 x 1010 km3
Average radius: 6,371.0 km
Average diameter: 12,742 km
Mean density: 5.515 g/cm3
Escape velocity: 11.186 km/s
Surface gravity: 9.798 m/s2
Visual magnitude: -3.86
Natural satellites: 1
Rings? – No
Semimajor axis: 149,600,000 km
Orbit period: 365.256 days
Perihelion: 147,090,000 km
Aphelion: 152,100,000 km
Mean orbital velocity: 29.78 km/s
Maximum orbital velocity: 30.29 km/s
Minimum orbital velocity: 29.29 km/s
Orbit inclination: 0.00°
Orbit eccentricity: 0.0167
Sidereal rotation period: 23.9345 hours
Length of day: 24.0000 hours
Axial tilt: 23.45°

Mass: 0.64185 x 1024 kg
Volume: 16.318 x 1010 km3
Average radius: 3,389.5 km
Average diameter: 6,779 km
Mean density: 3.933 g/cm3
Escape velocity: 5.03 km/s
Surface gravity: 3.71 m/s2
Visual magnitude: -1.52
Natural satellites: 2
Rings? – No
Semimajor axis: 227,920,000 km
Orbit period: 686.980 days
Perihelion: 206,620,000 km
Aphelion: 249,230,000 km
Mean orbital velocity: 24.13 km/s
Orbit inclination: 1.850°
Orbit eccentricity: 0.0935
Sidereal rotation period: 24.6229 hours
Length of day: 24.6597 hours
Axial tilt: 25.19 °
Discovery: Known since prehistoric times
Minimum distance from Earth: 55,700,000 km
Maximum distance from Earth: 401,300,000 km
Maximum apparent diameter from Earth: 25.1 arc seconds
Minimum apparent diameter from Earth: 3.5 arc seconds
Maximum visual magnitude: -2.91

Mass: 1,898.6 x 1024 kg
Volume: 143,128 x 1010 km3
Average radius: 69,911 km
Average diameter: 139,822 km
Mean density: 1.326 g/cm3
Escape velocity: 59.5 km/s
Surface gravity: 24.79 m/s2
Natural satellites: 63
Rings? – Yes
Semimajor axis: 778,570,000 km
Orbit period: 4,332.589 days
Perihelion: 740,520,000 km
Aphelion: 816,620,000 km
Mean orbital velocity: 13.07 km/s
Orbit inclination: 1.304°
Orbit eccentricity: 0.0489
Sidereal rotation period: 9.9250 hours
Length of day: 9.9259 hours
Axial tilt: 3.13°
Discovery: Known since prehistoric times
Minimum distance from Earth: 588,500,000 km
Maximum distance from Earth: 968,100,000 km
Maximum apparent diameter from Earth: 50.1 arc seconds
Minimum apparent diameter from Earth: 29.8 arc seconds
Maximum visual magnitude: -2.94

Mass: 568.46 x 1024 kg
Volume: 82,713 x 1010 km3
Average radius: 58,232 km
Average diameter: 116,464 km
Mean density: 0.687 g/cm3
Escape velocity: 35.5 km/s
Surface gravity: 10.44 m/s2
Natural satellites: 60
Rings? – Yes
Semimajor axis: 1,433,530,000 km
Orbit period: 10,759.22 days
Perihelion: 1,352,550,000 km
Aphelion: 1,514,500,000 km
Mean orbital velocity: 9.69 km/s
Orbit inclination: 2.485°
Orbit eccentricity: 0.0565
Sidereal rotation period: 10.656 hours
Length of day: 10.656 hours
Axial tilt: 26.73°
Discovery: Known since prehistoric times
Minimum distance from Earth: 1,195,500,000 km
Maximum distance from Earth: 1,658,500,000 km
Maximum apparent diameter from Earth: 20.1 arc seconds
Minimum apparent diameter from Earth: 14.5 arc seconds
Maximum visual magnitude: 0.43

Mass: 86.832 x 1024 kg
Volume: 6,833 x 1010 km3
Average radius: 25,362 km
Average diameter: 50,724 km
Mean density: 1.270 g/cm3
Escape velocity: 21.3 km/s
Surface gravity: 8.87 m/s2
Natural satellites: 27
Rings? – Yes
Semimajor axis: 2,872,460,000 km
Orbit period: 30,685.4 days
Perihelion: 2,741,300,000 km
Aphelion: 3,003,620,000 km
Mean orbital velocity: 6.81 km/s
Orbit inclination: 0.772°
Orbit eccentricity: 0.0457
Sidereal rotation period: 17.24 hours
Length of day: 17.24 hours
Axial tilt: 97.77°
Discovery: 13 March 1781
Minimum distance from Earth: 2,581,900,000 km
Maximum distance from Earth: 3,157,300,000 km
Maximum apparent diameter from Earth: 4.1 arc seconds
Minimum apparent diameter from Earth: 3.3 arc seconds
Maximum visual magnitude: 5.32

Mass: 102.43 x 1024 kg
Volume: 6,254 x 1010 km3
Average radius: 24,622 km
Average diameter: 49,244 km
Mean density: 1.638 g/cm3
Escape velocity: 23.5 km/s
Surface gravity: 11.15 m/s2
Natural satellites: 13
Rings? – Yes
Semimajor axis: 4,495,060,000 km
Orbit period: 60,189 days
Perihelion: 4,444,450,000 km
Aphelion: 4,545,670,000 km
Mean orbital velocity: 5.43 km/s
Orbit inclination: 1.769°
Orbit eccentricity: 0.0113
Sidereal rotation period: 16.11 hours
Length of day: 16.11 hours
Axial tilt: 28.32°
Discovery: 23 September 1846
Minimum distance from Earth: 4,305,900,000 km
Maximum distance from Earth: 4,687,300,000 km
Maximum apparent diameter from Earth: 2.4 arc seconds
Minimum apparent diameter from Earth: 2.2 arc seconds
Maximum visual magnitude: 7.78

We’ve written many articles about the Solar System. Here’s an article about how many moons there are in the Solar System, and here’s an article about the formation of the Solar System.

If you’d like more info on the Solar System, check out NASA’s Planetary Fact Sheet.

We’ve recorded several episodes of Astronomy Cast about the Solar System. Start here, Episode 49: Mercury.

Tenth Planet: The Next World in the Solar System

Tenth planet? Artists concept of the view from Eris with Dysnomia in the background, looking back towards the distant sun. Credit: Robert Hurt (IPAC)
Tenth planet? Artists concept of the view from Eris with Dysnomia in the background, looking back towards the distant sun. Credit: Robert Hurt (IPAC)

Before 1930, there were 8 planets in the Solar System. And then with the discovery of Pluto in 1930, the total number of planets rose to 9. Although astronomers kept searching for more planets, it wasn’t until 2005 that an object larger than Pluto was found orbiting in the distant Solar System. This new object was known as Eris, and many considered it to be a tenth planet; but it actually created a controversy that ended up with Pluto being kicked out of the planet club and becoming a dwarf planet. There really is no 10th planet, in fact, we don’t even have a ninth planet any more.

Discovery of Eris

Eris, originally named 2003 ub 313 was discovered by Palomar observatory researcher Mike Brown; Mike has been behind many of the trans-Neptunian discoveries in the last decade. Mike and his team discovered Eris by systematically scanning the sky for objects moving at the right speed in the right object to be in the outer Solar System.

Further observations of Eris showed that it was actually larger than Pluto by a significant amount; it measured 2,500 km across, compared to Pluto’s 2,300 km diameter. And it orbited at a distance of 67 astronomical units, compared to Pluto’s 39 AU (1 AU is the average distance from the Earth to the Sun).

Tenth Planet, Dwarf Planet

Because there was now a larger object than Pluto found orbiting the Sun, astronomers needed to decide whether this would be come the tenth planet. At a meeting of the International Astronomical Union in 2006, astronomers decided to redefine their classification of a planet. And these new rules excluded Eris. Instead of becoming the tenth planet, Eris became a dwarf planet; the same fate as Pluto.

We’ve written many articles about Eris for Universe Today. Here’s an article about how Eris is changing, and here’s an article about how Xena was renamed to Eris.

If you’d like more info on Eris, check out NASA’s page on Eris.

We’ve also recorded an episode of Astronomy Cast that explains why Pluto isn’t a planet any more. Listen here, Episode 1: Pluto’s Planetary Identity Crisis.