Vacations can be quite enjoyable. Visiting historic cities, lounging in the Sun on a tropical beach, or snuggling up at a cozy mountain resort. But while the destinations are great, traveling itself can be a chore. Crowds, cramped flights, delays. It would be great if there were some short cut to our destination. Now imagine the vacationers of a galactic empire. It’s great to visit the diamond shores of Exoticon 5, but nobody enjoys all that mucking about in hyperspace. So why not bring these worlds closer to home? That’s the idea behind a study recently published in MNRAS. It basically looks at how a super-advanced civilization might pack a bunch of planets into the habitable zone of a single star.Continue reading “Building Planetary Systems That Could Last Forever”
Imagine trying to study an object light-years away that is less than 20 kilometers in diameter. The object is so dense that it’s made of material that can’t exist naturally on Earth. This is the challenge astronomers face when studying neutron stars, so they have to devise ingenious ways to do it. Recently a team figured out how to study them by using the power of resonance.Continue reading “A new way to see Inside Neutron Stars”
200 light-years away from Earth, there’s a K-type main-sequence star named TOI (TESS Object of Interest) 178. When Adrian Leleu, an astrophysicist at the Center for Space and Habitability of the University of Bern, observed it, it appeared to have two planets orbiting it at roughly the same distance. But that turned out to be incorrect. In fact, six exoplanets orbit the smallish star.
And five of those six are locked into an unexpected orbital configuration.Continue reading “Exoplanetary System Found With 6 Worlds in Orbital Resonance”
Jupiter is the Boss.
Well, in terms of planets in our Solar System it is. It’s played a huge role in shaping the Solar System due to its mass and its gravity. Here’s a few ways it’s shaped our system:Continue reading “Astronomers Find a Planet With Three Times the Mass of Jupiter”
To date, astronomers have confirmed the existence of 4,152 extrasolar planets in 3,077 star systems. While the majority of these discoveries involved a single planet, several hundred star systems were found to be multi-planetary. Systems that contain six planets or more, however, appear to be rarer, with only a dozen or so cases discovered so far.
This is what astronomers found after observing HD 158259, a Sun-like star located about 88 light-years from Earth, for the past seven years using the SOPHIE spectrograph. Combined with new data from the Transiting Exoplanet Space Satellite (TESS), an international team reported the discovery of a six planet system where all were in near-perfect rhythm with each other.Continue reading “Astronomers Find a Six-Planet System Which Orbit in Lockstep With Each Other”
Like a long-married couple accustomed to each other’s kitchen habits, two of Neptune’s moons are masters at sharing space without colliding. And though both situations may appear odd to an observer, there’s a certain dance-like quality to them both.Continue reading “Two of Neptune’s Moons Dance Around Each Other as they Orbit”
With the dawning of the Space Age in the 1950s, human beings were no longer confined to studying the Solar planets and other astronomical bodies with Earth-based instruments alone. Instead crewed missions have gone into orbit and to the Moon while robotic missions have traveled to every corner of the Solar System. And in the process, we have learned some interesting things about the planets, planetoids, and asteroids in our Solar neighborhood.
For example, we have learned that all the Solar planets have their own particular patterns and cycles. For instance, even though Mercury is an airless body, it does have a tenuous exosphere and experiences seasons of a sort. And while it is known for being extremely hot, it also experiences extremes of cold, to the point that ice can exist on its surface. While it is by no means what we are used to here on Earth, Mercury still experiences a kind of “weather”.
As noted, Mercury has no atmosphere to speak of, owing to its small size and extremes in temperature. However, it does have a tenuous and variable exosphere that is made up of hydrogen, helium, oxygen, sodium, calcium, potassium and water vapor, with a combined pressure level of about 10-14 bar (one-quadrillionth of Earth’s atmospheric pressure).
It is believed this exosphere was formed from particles captured from the Sun (i.e solar wind) as well as volcanic outgassing and debris kicked into orbit by micrometeorite impacts. In any case, Mercury’s lack of a viable atmosphere is the reason why it is unable to retain heat from the Sun, which leads to extreme variations between night and day for the rocky planet.
Mercury’s temperature variations are also attributed to its orbital eccentricity of 0.2056, which is the most extreme of any planet in the Solar System. Essentially, its distance from the Sun ranges from 46 million km (29 million mi) at its closest (perihelion) to 70 million km (43 million mi) at its farthest (aphelion). As a result, the side facing the Sun reaches temperatures of up to 700 K (427° C), the side in shadow dips down to 100 K (-173° C).
With an average rotational speed of 10.892 km/h (6.768 mph), Mercury also takes 58.646 days to complete a single rotation. This means that Mercury has a spin-orbit resonance of 3:2, where it completes three rotations on its axis for every two rotations completed around the Sun. This does not, however, mean that three days last the same as two years on Mercury.
In fact, its high eccentricity and slow rotation mean that it takes 176 Earth days for the Sun to return to the same place in the sky (aka. a solar day). In short, a single day on Mercury is twice as long as a single 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 (the second smallest).
This low tilt means that the polar regions are constantly in shadow, which leads to another interesting feature about Mercury. Yes, despite how hot its Sun-facing side can become, the existence of water ice and even organic molecules have been confirmed on Mercury’s surface. But this only true at the poles, where the floors of deep craters are never exposed to direct sunlight, and temperatures within them therefore remain below the planetary average.
These icy regions are believed to contain about 1014–1015 kg (1 to 10 billion metric tons, 1.1 to 11 billion US tons) of frozen water, and may be covered by a layer of regolith that inhibits sublimation. The origin of the ice on Mercury is not yet known, but the two most likely sources are from outgassing of water from the planet’s interior or deposition by the impacts of comets.
When one talks about the “weather” on Mercury, they are generally confined to talking about variations between the Sun-facing side and the night side. Over the course of two years, that weather will remain scorching hot or freezing cold. In that respect, we could say that a single season on Mercury lasts a full four years, and includes a “Midnight Sun” that lasts two years, and a “Polar Night” that lasts the same.
Between its rapid and very eccentric orbit, its slow rotation, and its strange diurnal and annual patterns, Mercury is a very extreme planet with a very extreme environment. It only makes sense that its weather would be similarly extreme. Hey, there’s a reason nobody lives there, at least not yet…
We have written many interesting articles about the weather on other planets here at Universe Today. Here’s What is the Weather like on Venus?, What is the Weather Like on Mars?, What is the Weather Like on Jupiter?, What is the Weather Like on Saturn?, What is the Weather Like on Uranus?, and What is the Weather Like on Neptune?
We’ve also recorded an entire episode of Astronomy Cast all about Mercury. Listen here, Episode 49: Mercury.
Mercury was appropriately named after the Roman messenger of the Gods. This is owed to the fact that its apparent motion in the night sky was faster than that of any of the other planets. As astronomers learned more about this “messenger planet”, they came to understand that its motion was due to its close orbit to the Sun, which causes it to complete a single orbit every 88 days.
Mercury’s proximity to the Sun is merely one of its defining characteristics. Compared to the other planets of the Solar System, it experiences severe temperature variations, going from very hot to very cold. It’s also very rocky, and has no atmosphere to speak of. But to truly get a sense of how Mercury stacks up compared to the other planets of the Solar System, we need to a look at how Mercury compares to Earth.
Size, Mass and Orbit:
The diameter of Mercury is 4,879 km, which is approximately 38% the diameter of Earth. In other words, if you put three Mercurys side by side, they would be a little larger than the Earth from end to end. While this makes Mercury smaller than the largest natural satellites in our system – such as Ganymede and Titan – it is more massive and far more dense than they are.
In fact, Mercury’s mass is approximately 3.3 x 1023 kg (5.5% the mass of Earth) which means that its density – at 5.427 g/cm3 – is the second highest of any planet in the Solar System, only slightly less than Earth’s (5.515 g/cm3). This also means that Mercury’s surface gravity is 3.7 m/s2, which is the equivalent of 38% of Earth’s gravity (0.38 g). This means that if you weighed 100 kg (220 lbs) on Earth, you would weigh 38 kg (84 lbs) on Mercury.
Meanwhile, the surface area of Mercury is 75 million square kilometers, which is approximately 10% the surface area of Earth. If you could unwrap Mercury, it would be almost twice the area of Asia (44 million square km). And the volume of Mercury is 6.1 x 1010 km3, which works out to 5.4% the volume of Earth. In other words, you could fit Mercury inside Earth 18 times over and still have a bit of room to spare.
In terms of orbit, Mercury and Earth probably could not be more different. For one, Mercury has the most eccentric orbit of any planet in the Solar System (0.205), compared to Earth’s 0.0167. Because of this, its distance from the Sun varies between 46 million km (29 million mi) at its closest (perihelion) to 70 million km (43 million mi) at its farthest (aphelion).
This puts Mercury much closer to the Sun than Earth, which orbits at an average distance of 149,598,023 km (92,955,902 mi), or 1 AU. This distance ranges from 147,095,000 km (91,401,000 mi) to 152,100,000 km (94,500,000 mi) – 0.98 to 1.017 AU. And with an average orbital velocity of 47.362 km/s (29.429 mi/s), it takes Mercury a total 87.969 Earth days to complete a single orbit – compared to Earth’s 365.25 days.
However, since Mercury also takes 58.646 days to complete a single rotation, it takes 176 Earth days for the Sun to return to the same place in the sky (aka. a solar day). So on Mercury, a single day is twice as long as a single year. Meanwhile on Earth, a single solar day is 24 hours long, owing to its rapid rotation of 1674.4 km/h. Mercury also has the lowest axial tilt of any planet in the Solar System – approximately 0.027°, compared to Earth’s 23.439°.
Structure and Composition:
Much like Earth, Mercury is a terrestrial planet, which means it is composed of silicate minerals and metals that are differentiated between a solid metal core and a silicate crust and mantle. For Mercury, the breakdown of these elements is higher than Earth. Whereas Earth is primarily composed of silicate minerals, Mercury is composed of 70% metallic and 30% of silicate materials.
Also like Earth, Mercury’s interior is believed to be composed of a molten iron that is surrounded by a mantle of silicate material. Mercury’s core, mantle and crust measure 1,800 km, 600 km, and 100-300 km thick, respectively; while Earth’s core, mantle and crust measure 3478 km, 2800 km, and up to 100 km thick, respectively.
What’s more, geologists estimate that Mercury’s core occupies about 42% of its volume (compared to Earth’s 17%) and the core has a higher iron content than that of any other major planet in the Solar System. Several theories have been proposed to explain this, the most widely accepted being that Mercury was once a larger planet that was struck by a planetesimal that stripped away much of the original crust and mantle.
In terms of its surface, Mercury is much more like the Moon than Earth. It has a dry landscape pockmarked by asteroid impact craters and ancient lava flows. Combined with extensive plains, these indicate that the planet has been geologically inactive for billions of years.
Names for these features come from a variety of sources. Craters are named for artists, musicians, painters, and authors; ridges are named for scientists; depressions are named after works of architecture; mountains are named for the word “hot” in different languages; planes are named for Mercury in various languages; escarpments are named for ships of scientific expeditions, and valleys are named after radio telescope facilities.
During and following its formation 4.6 billion years ago, Mercury was heavily bombarded by comets and asteroids, and perhaps again during the Late Heavy Bombardment period. Due to its lack of an atmosphere and precipitation, these craters remain intact billions of years later. Craters on Mercury range in diameter from small bowl-shaped cavities to multi-ringed impact basins hundreds of kilometers across.
The largest known crater is Caloris Basin, which measures 1,550 km (963 mi) in diameter. The impact that created it was so powerful that it caused lava eruptions on the other side of the planet and left a concentric ring over 2 km (1.24 mi) tall surrounding the impact crater. Overall, about 15 impact basins have been identified on those parts of Mercury that have been surveyed.
Earth’s surface, meanwhile, is significantly different. For starters, 70% of the surface is covered in oceans while the areas where the Earth’s crust rises above sea level forms the continents. Both above and below sea level, there are mountainous features, volcanoes, scarps (trenches), canyons, plateaus, and abyssal plains. The remaining portions of the surface are covered by mountains, deserts, plains, plateaus, and other landforms.
Mercury’s surface shows many signs of being geologically active in the past, mainly in the form of narrow ridges that extend up to hundreds of kilometers in length. It is believed that these were formed as Mercury’s core and mantle cooled and contracted at a time when the crust had already solidified. However, geological activity ceased billions of years ago and its crust has been solid ever since.
Meanwhile, Earth is still geologically active, owning to convection of the mantle. The lithosphere (the crust and upper layer of the mantle) is broken into pieces called tectonic plates. These plates move in relation to one another and interactions between them is what causes earthquakes, volcanic activity (such as the “Pacific Ring of Fire“), mountain-building, and oceanic trench formation.
Atmosphere and Temperature:
When it comes to their atmospheres, Earth and Mercury could not be more different. Earth has a dense atmosphere composed of five main layers – the Troposphere, the Stratosphere, the Mesosphere, the Thermosphere, and the Exosphere. Earth’s atmosphere is also primarily composed of nitrogen (78%) and oxygen (21%) with trace concentrations of water vapor, carbon dioxide, and other gaseous molecules.
Because of this, the average surface temperature on Earth is approximately 14°C, with plenty of variation due to geographical region, elevation, and time of year. The hottest temperature ever recorded on Earth was 70.7°C (159°F) in the Lut Desert of Iran, while the coldest temperature was -89.2°C (-129°F) at the Soviet Vostok Station on the Antarctic Plateau.
Mercury, meanwhile, has a tenuous and variable exosphere that is made up of hydrogen, helium, oxygen, sodium, calcium, potassium and water vapor, with a combined pressure level of about 10-14 bar (one-quadrillionth of Earth’s atmospheric pressure). It is believed this exosphere was formed from particles captured from the Sun, volcanic outgassing and debris kicked into orbit by micrometeorite impacts.
Because it lacks a viable atmosphere, Mercury has no way to retain the heat from the Sun. As a result of this and its high eccentricity, the planet experiences far more extreme variations in temperature than Earth does. Whereas the side that faces the Sun can reach temperatures of up to 700 K (427° C), the side that is in darkness can reach temperatures as low as 100 K (-173° C).
Despite these highs in temperature, the existence of water ice and even organic molecules has been confirmed on Mercury’s surface. The floors of deep craters at the poles are never exposed to direct sunlight, and temperatures there remain below the planetary average. In this respect, Mercury and Earth have something else in common, which is the presence of water ice in its polar regions.
Much like Earth, Mercury has a significant, and apparently global, magnetic field, one which is about 1.1% the strength of Earth’s. It is likely that this magnetic field is generated by a dynamo effect, in a manner similar to the magnetic field of Earth. This dynamo effect would result from the circulation of the planet’s iron-rich liquid core.
Mercury’s magnetic field is strong enough to deflect the solar wind around the planet, thus creating a magnetosphere. The planet’s magnetosphere, though small enough to fit within Earth, is strong enough to trap solar wind plasma, which contributes to the space weathering of the planet’s surface.
All told, Mercury and Earth are in stark contrast. While both are terrestrial in nature, Mercury is significantly smaller and less massive than Earth, though it has a similar density. Mercury’s composition is also much more metallic than that of Earth, and its 3:2 orbital resonance results in a single day being twice as long as a year.
But perhaps most stark of all are the extremes in temperature variations that Mercury goes through compared to Earth. Naturally, this is due to the fact that Mercury orbits much closer to the Sun than the Earth does and has no atmosphere to speak of. And its long days and long nights also mean that one side is constantly being baked by the Sun, or in freezing darkness.
We have written many stories about Mercury on Universe Today. Here’s Interesting Facts About Mercury, What Type of Planet is Mercury?, How Long is a Day on Mercury?, The Orbit of Mercury. How Long is a Year on Mercury?, What is the Surface Temperature of Mercury?, Water Ice and Organics Found at Mercury’s North Pole, Characteristics of Mercury,, Surface of Mercury, and Missions to Mercury
We have also recorded a whole episode of Astronomy Cast that’s just about planet Mercury. Listen to it here, Episode 49: Mercury.
The extra-solar planet known as Proxima b has occupied a special place in the public mind ever since its existence was announced in August of 2016. As the closest exoplanet to our Solar System, its discovery has raised questions about the possibility of exploring it in the not-too-distant future. And even more tantalizing are the questions relating to its potential habitability.
Despite numerous studies that have attempted to indicate whether the planet could be suitable for life as we know it, nothing definitive has been produced. Fortunately, a team of astrophysics from the University of Exeter – with the help of meteorology experts from the UK’s Met Office – have taken the first tentative steps towards determining if Proxima b has a habitable climate.
According to their study, which appeared recently in the journal Astronomy & Astrophysics, the team conducted a series of simulations using the state-of-the-art Met Office Unified Model (UM). This numerical model has been used for decades to study Earth’s atmosphere, with applications ranging from weather prediction to the effects of climate change.
With this model, the team simulated what the climate of Proxima b would be like if it had a similar atmospheric composition to Earth. They also conducted simulations on what the planet would be like it if had a much simpler atmosphere – one composed of nitrogen with trace amounts of carbon dioxide. Last, but not least, they made allowances for variations in the planet’s orbit.
For instance, given the planet’s distance from its sun – 0.05 AU (7.5 million km; 4.66 million mi) – there have been questions about the planet’s orbital characteristics. On the one hand, it could be tidally-locked, where one face is constantly facing towards Proxima Centauri. On the other, the planet could be in a 3:2 orbital resonance with its sun, where it rotates three times on its axis for every two orbits (much like Mercury experiences with our Sun).
In either case, this would result in one side of the planet being exposed to quite a bit of radiation. Given the nature of M-type red dwarf stars, which are highly variable and unstable compared to other types of stars, the sun-facing side would be periodically irradiated. Also, in both orbital scenarios, the planet would be subject to significant variations in temperature that would make it difficult for liquid water to exist.
For example, on a tidally-locked planet, the main atmospheric gases on the night-facing side would be likely to freeze, which would leave the daylight zone exposed and dry. And on a planet with a 3:2 orbital resonance, a single solar day would most likely last a very long time (a solar day on Mercury lasts 176 Earth days), causing one side to become too hot and dry the other side too cold and dry.
By taking all this into account, the team’s simulations allowed for some crucial comparisons with previous studies, but also allowed the team to reach beyond them. As Dr. Ian Boutle, an Honorary University Fellow at the University of Exeter and the lead author of the paper, explained in a University press release:
“Our research team looked at a number of different scenarios for the planet’s likely orbital configuration using a set of simulations. As well as examining how the climate would behave if the planet was ‘tidally-locked’ (where one day is the same length as one year), we also looked at how an orbit similar to Mercury, which rotates three times on its axis for every two orbits around the sun (a 3:2 resonance), would affect the environment.”
In the end, the results were quite favorable, as the team found that Proxima b would have a remarkably stable climate with either atmosphere and in either orbital configuration. Essentially, the UM software simulations showed that when both atmospheres and both the tidally-locked and 3:2 resonance configurations were accounted for, there would still be regions on the planet where water was able to exist in liquid form.
Naturally, the 3:2 resonance example resulted in more substantial areas of the planet falling within this temperature range. They also found that an eccentric orbit, where the distance between the planet and Proxima Centauri varied to a significant degree over the course of a single orbital period, would lead to a further increase in potential habitability.
As Dr James Manners, another Honorary University Fellow and one of the co-authors on the paper, said:
“One of the main features that distinguishes this planet from Earth is that the light from its star is mostly in the near infra-red. These frequencies of light interact much more strongly with water vapor and carbon dioxide in the atmosphere which affects the climate that emerges in our model.”
Of course, much more work needs be done before we can truly understand whether this planet is capable of supporting life as we know it. Beyond feeding the hopes of those who would like to see it colonized someday, studies into Proxima b’s conditions are also of extreme importance in determining whether or not indigenous life exists there right now.
But in the meantime, studies such as this are extremely helpful when it comes to anticipating what kinds of environments we might find on distant planets. Dr Nathan Mayne – the scientific lead on exoplanet modelling at the University of Exeter and a co-author on the paper – also indicated that climate studies of this kind could have applications for scientists here at home.
“With the project we have at Exeter we are trying to not only understand the somewhat bewildering diversity of exoplanets being discovered, but also exploit this to hopefully improve our understanding of how our own climate has and will evolve,” he said. What’s more, it helps to illustrate how conditions here on Earth can be used to predict what may exist in extra-solar environments.
While that might sound a bit Earth-centric, it is entirely reasonable to assume that planets in other star systems are subject to processes and mechanics similar to what we’ve seen on the Solar planets. And this is something we are invariably forced to do when it comes to searching for habitable planets and life beyond our Solar System. Until we can go there directly, we will be forced to measure what we don’t know by what we do.
Mercury is one of the most unusual planets in our Solar System, at least by the standards of us privileged Earthlings. Despite being the closest planet to our Sun, it is not the hottest (that honor goes to Venus). And because of its virtually non-existence atmosphere and slow rotation, temperatures on its surface range from being extremely hot to extremely cold.
Equally unusual is the diurnal cycle on Mercury – i.e. the cycle of day and night. A single year lasts only 88 days on Mercury, but thanks again to its slow rotation, a day lasts twice as long! That means that if you could stand on the surface of Mercury, it would take a staggering 176 Earth days for the Sun to rise, set and rise again to the same place in the sky just once!
Distance and Orbital Period:
Mercury is the closest planet to our Sun, but it also has the most eccentric orbit (0.2056) of any of the Solar Planets. This means that while its average distance (semi-major axis) from the Sun is 57,909,050 km (35,983,015 mi) or 0.387 AUs, this ranges considerably – from 46,001,200 km (2,8583,820 mi) at perihelion (closet) to 69,816,900 km (43,382,210 mi) at aphelion (farthest).
Because of this proximity, Mercury has a rapid orbital period, which varies depending on where it is in its orbit. Naturally, it moves fastest when it is at its closest to the Sun, and slowest when it is farthest. On average, its orbital velocity is 47.362 km/s (29.43 mi/s), which means it takes only 88 days to complete a single orbit of the Sun.
Astronomers used to suspect that Mercury was tidally locked to the Sun, meaning that it always showed the same face to the Sun – similar to how the Moon is tidally locked to the Earth. But radar-Doppler measurements obtained in 1965 demonstrated that Mercury is actually rotating very slowly compared to the Sun.
Sidereal vs. Solar Day:
Based on data obtained by these radar measurements, Mercury is now known to be in 3:2 orbital resonance with the Sun. This means that the planet completes three rotations on its axis for every two orbits it makes around the Sun. At it’s current rotational velocity – 3.026 m/s, or 10.892 km/h (6.77 mph) – it takes Mercury 58.646 days to complete a single rotation on its axis.
While this might lead some to conclude that a single day on Mercury is about 58 Earth days – thus making the length of a day and year correspond to the same 3:2 ratio – this would be inaccurate. Due to its rapid orbital velocity and slow sidereal rotation, a Solar Day on Mercury (the time it takes for the Sun to return to the same place in the sky) is actually 176 days.
In that respect, the ratio of days to years on Mercury is actually 1:2. The only places that are exempt to this day and night cycle are the polar regions. The cratered northern polar region, for example, exists in a state of perpetual shadow. Temperatures in these craters are also cool enough that significant concentrations of water ice can exist in stable form.
For over 20 years, scientists believed that radar-bright images from Mercury’s northern polar regions might indicate the presence of water ice there. In November of 2012, NASA’s MESSENGER probe examined the northern polar region using its neutron spectrometer and laser altimeter and confirmed the presence of both water ice and organic molecules.
Yes, as if Mercury weren’t strange enough, it turns out that a single day on Mercury lasts as long as two years! Just another oddity for a planet that likes to keep things really hot, really cold, and is really eccentric.
We’ve written many articles about Mercury for Universe Today. Here’s How Long is Day on the Other Planets?, Which Planet has the Longest Day?, How Long is a Day on Venus?, How Long is a Day on Earth?, How Long is a Day on the Moon?, How Long is a Day on Mars?, How Long is a Day on Jupiter?, How Long is a Day on Saturn?, How Long is a Day on Uranus?, How Long is a Day on Neptune?, and How Long is a Day on Pluto?
We’ve also recorded an entire episode of Astronomy Cast all about Mercury. Listen here, Episode 49: Mercury.