Are there any planets or moons denser than Earth?

Are there any planets or moons denser than Earth?

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Earth has the highest density out of all planets, planetoids and moons of our planetary system, and also has a higher density than the Sun. Do we know any exoplanets or moons denser than Earth?

From the Wikipedia page on Chthonian planet:

Transit-timing variation measurements indicate for example that Kepler-52b, Kepler-52c and Kepler-57b have maximum-masses between 30 and 100 times the mass of Earth (although the actual masses could be much lower); with radii about 2 Earth radii, they might have densities larger than that of an iron planet of the same size. These exoplanets are orbiting very close to their stars and could be the remnant cores of evaporated gas giants or brown dwarfs. If cores are massive enough they could remain compressed for billions of years despite losing the atmospheric mass.

From another question on the This Site I just found out about an exoplanet with a much higher density than Earth: Kepler-131c.

I feel it's a cheap answer but heavy Jupiters can get much denser than Earth because planets with Jupiter's mass stop adding size as they add more mass. A planet with Jupiter's size and 10-12 times Jupiter's mass would be over twice Earth's density.

As far as Earth-like planets, there's a cool property of terrestrial planets, more mass means more tightly packed in their cores. Basically a similar effect to the heavy Jupiters but not as pronounced. If you double the mass and keep the element ratio the same, the density should increase. For example, a planet like Mercury with a very high iron content, but much greater mass should easily surpass Earth's density.

Kepler 10b, appears to be a super-earth and its estimated density is greater than Earth's at 5.8 ± 0.8 g/cm3. It has a mass of about 3.7 Earths. The ± 0.8 g/cm3 offers some room for uncertainty, but some of the more massive terrestrial planets should be more dense than Earth.

I'd take this list with a grain of salt, but you can sort by density.

Kepler 131c which you mention appears to be a super Earth but its mass has a high margin for error. I would add that a mass 8 times that of Earth and a radius smaller than Earth is probably impossible, so I'm highly skeptical of some of those numbers.

The Earth / Moon System

We are going to begin this lesson with a study of some of the properties of the Earth. We are going to focus on some of the physical processes that occur on Earth, and then we are going to discuss how the other inner planets differ from our observations of the Earth. We will emphasize the interior, the crust, and the atmosphere.


The Earth has an atmosphere that consists of a mix of various gases (roughly 80% nitrogen and 20% oxygen, with small traces of argon, carbon dioxide, water vapor, and others) that is retained by the Earth's gravity. The Moon has no atmosphere today because its gravitational field is weaker than Earth's, and it wasn't strong enough to retain whatever primordial gases were there originally. The atmosphere helps to regulate the temperature on the Earth, keeping it within a fairly narrow range from day to night. On the Moon, the temperature can vary by 300 degrees during one Moon day — it is hotter than boiling water when the Sun is directly overhead on the Moon and is much colder than water's freezing point on the night side of the Moon.

The temperature on Earth is influenced by the Greenhouse Effect (this name comes from an analogy that isn't perfect, but is close enough for our purposes). This works as follows:

  • The Earth absorbs visible light from the Sun.
  • The Earth's surface heats up and radiates light in the infrared part of the spectrum.
  • Water vapor and carbon dioxide (primarily) in the atmosphere absorb the infrared light from the surface, heating up the atmosphere.
  • The atmosphere radiates infrared radiation at the surface of the Earth, also heating it up.

When this process is in equilibrium, the temperature of the Earth varies around a mean temperature from year to year. The discussion about global warming arises from the observation that the increase in carbon dioxide in the atmosphere (mostly due to humans burning fossil fuels) correlates with the steady increase in the mean temperature of the Earth because of an increase in the magnitude of the greenhouse effect.

Apart from the fact that we breathe the gas in the Earth's atmosphere and that the atmosphere keeps the temperature of the Earth warm enough for us to exist, the atmosphere also has played a role in shaping the Earth's surface. We do not see very many craters on the Earth, although there are a few, including "Meteor Crater," or "Barringer Crater," in Arizona.

On the Earth, the atmosphere protects us from the impacts of some objects, but the weather also erodes craters over time, making them no longer visible. Satellite images reveal the existence of some ancient craters on Earth, including Manicouagan Crater.

Try this!

Using the free application "Google Earth," you can tour the Meteor Crater and Manicouagan Crater from Google Earth to get a sense of their real scale. Can you find any other craters on Earth using Google Earth? If you want to skip right to the answer, you can see the database of currently identified craters in the "Earth Impact Database".

You can compare the Google Earth images of the surface of the Earth to the Google Moon images of the surface of the Moon (Note: In the Google Earth application, you can now also bring up the 3D map of the Moon instructions are available on the Google Moon website). How does the appearance of the Moon's surface differ from Earth's?

The Moon is very different. Since it lacks an atmosphere, most of the impact craters that formed on its surface are still visible today, and they will never erode away like those on Earth.

Crust and Interior

When we discuss the surface and the structure of the planets, we are going to focus on their density, because this property tells us a lot about the composition of the planet. Recall, the density is the mass of an object divided by its volume. If you consider comparing a cube of sugar to a cube of lead that is the same size, you will expect the cube of lead to feel heavier than the cube of sugar. This is because in the same volume (a cube of the same size), the lead will contain more mass than the sugar. The metric system was designed so that one cubic centimeter of water has a mass of 1 gram, so we say that the density of water is 1 gram per cubic centimeter (or equivalently, 1000 kilograms per cubic meter, which is another popular unit of measurement for density). The rocks and metals that we are familiar with are all denser than water. A typical rock has a density of about 3 g / cm 3 and a typical metal (say iron) has a density of about 8 g / cm 3 .

Our measurements of the Earth's mass (from Kepler's Laws) and its volume (from its measured size) tell us that its average density is about 5.5g / cm 3 — about half way between rock and iron. By studying data from earthquakes around the world, geologists have determined that the Earth is not uniformly made up of the same material. Instead it is differentiated into different layers. We live on the crust. Below the crust is the mantle, and below the mantle is a core. The crust is about 3 g / cm 3 and the inner core is about 12 g / cm 3 , which suggest the crust is made up mostly of ordinary rock, and the inner core is denser than the metal lead. The mantle is a rocky layer intermediate in density between the crust and the core, and it is semi-fluid, somewhat like the consistency of bubble gum or flexible plastic. The crust of the Earth sits on top of the mantle, and because of convection (a process which occurs in fluids with a temperature difference between different layers — the warmer fluid rises, cools off, and then sinks, creating a flow) in the mantle, the plates that make up the crust are very slowly moving with respect to each other. This process of plate tectonics is the cause of earthquakes, the source of mountains and trenches under the oceans, and is one you will discuss in much more detail in the other courses in this program (e.g., EARTH 501 and EARTH 520). For the purposes of ASTRO 801, you should keep in mind though, that on Earth, plate tectonics is ongoing. We will want to consider if this is true of the other objects in the Solar System as we compare and contrast the planets and moons.

The Moon is very different. The average density is 3.3 g / cm 3 , but more importantly, there is less variation in the density from core to crust than the Earth. The Moon is not geologically active — there are no volcanoes or plate tectonics on the Moon. Equipment left behind by the astronauts who landed on the Moon has detected Moonquakes, but they are very mild. Interestingly, it looks like the core in the Moon is off center. The crust is about 60 km thick on the near side (facing Earth), but it is 150 km thick on the far side. In 2014, my Penn State colleagues published a new study suggesting that this variation in the thickness of the Moon's crust was caused by the very hot, molten Earth heating the Earth-facing side while the far side of the Moon cooled off.

The surface of the Moon shows two very different regions. The dark regions (the Maria) are lowlands that were filled in by molten mantle material (lava) about 3.5 billion years ago. The light colored areas are highlands that are made of the original crust of the Moon. You can tell that the Maria are younger than the light colored regions because they have fewer craters. Originally, there were just as many craters in the Maria region as there are everywhere else, but when the molten rock filled in these regions and solidified, it covered many of them. Because the Moon has no atmosphere, the craters are very well preserved, giving us details on what the number of impacts was like in the early history of the Solar System. In the image below, you can see the Maria have few craters, while the other regions of the Moon are speckled with dark craters.

The far side of the Moon (the half that is never pointed towards Earth) looks very different from the side that we see there are no Maria. This is attributed to the difference in the crust thickness. Since the far side of the Moon is about twice as thick, Lava was not able to penetrate the crust to fill in the lowlands, solidify, and create Maria.

The current leading theory for the formation of the Moon suggests that it was created in a collision between the proto-Earth and a Mars-sized object. Computer simulations of a collision like this show that the material that breaks off of the proto-Earth should wind up coalescing into a Moon-sized object in orbit around the Earth. The material that created the Moon would have come from the crust and mantle of the proto-Earth, which explains its lower density than the Earth. There is an excellent visualization of this in the video "Cosmic Collisions", which was released by NASA and the American Museum of Natural History. AMNH appears to have removed all of the public links to the video, but the History Channel has a good, similar video that includes most of the details in the Cosmic Collisions video.

High gravity rocky planets?

I'm asking how much mass can a rocky planet have? (and still be rocky)
Guessing it's around 1.5x Earth diameter or 5x Earth Mass. More than 5x the gravity on the surface?

But what happens if a rocky planet goes beyond that? That range is no where near the mass of a brown dwrarf star..

Can such a planet have life (too hypthetical for now)?

#2 Avi

Consider that Neptune's rocky core is just over the mass of the earth,
I'd imagine some of the rocky "super earth" planets could, during formation gather more gases and as such become ice giants.

Keep in mind, gravity follows the inverse square law. Yes, if you had a planet the diameter of earth, but with five times the mass - you'd have roughly five times our surface gravity, and probably a super thick atmosphere. However, five earth masses AND five earth radii would not equal five times our surface gravity.

A planet such as your hypothetical one exists - it's called CoRoT-7b and might actually have been more similar to Neptune but lost most of it's mass, save for it's rocky core.

Another similar super earth candidate is GJ 1214 b - it is about 2.6 times earths diameter, has about six times earths mass, but has about 90% of earth's surface gravity.

#3 Erric

So it seems anything much larger than Earth will develop a super thick "gas giant" type atmosphere. Except if it's really close to a star like CoRoT-7b.

I was hoping some guy can bench-press on a heavy gravity planet or find some really tough bugs. But this doesn't seem possible. (even if the planet is small & very dense)

I found COROT-exo-3b while searching, which is closer to what I'm looking for. But it seems too hot to land on. (maybe not if it's far enough away from it's sun).

#4 Avi

I wouldn't say larger rocky cores produce a gas or ice giant for sure - all depends on the distance from the host star.

If the math is right GJ_1214_b could have temperatures that might not be inhospitable to a lander.

However, even if we found an exact twin of earth, say as close as a light year out - it would still be next to impossible for us to send anything there, based on current technology.

I'm sure, before too long, we're going to find something with a surface temp of just under 300K, and at JUST the right distance from it's parent star to support liquid water. There's got to be a few out there.

#5 Erric

#6 Jarad

Guessing it's around 1.5x Earth diameter or 5x Earth Mass. More than 5x the gravity on the surface?

Just a quick note: Assuming that by "rocky planet" you mean one with about the same density as the earth, then the mass will go up by the cube of the diameter, and the surface gravity will go up by the diameter. So a rocky planet with 1.5x the earth's diameter would have 3.375x the earth's mass, and 1.5G at the surface. A planet with 5x the earth's mass would be 1.71x the diameter and have 1.71G at the surface.

I don't know if there is some limit above which the planet would start to compress to significantly higher density than the earth. I suspect that would depend more on the content than the size, with more heavy elements = higher density. An increase in density can have a large effect on the surface gravity, since it results in increased mass and/or reduced radius, both of which will increase the surface gravity.

2 Answers 2

I was actually just reading a great What If? article on this found here. Flight on other planets is possible. I think the included comic strip summarizes it wonderfully:

As for each valid body in our solar system (barring Earth of course), I'm going to paraphrase a bit:

The Sun: Attempting flight on the sun is more or less useless as any vessel close enough to feel its atmosphere would be instantly vaporized.

Mars: The article goes into a lengthy discussion about simulation via X-Plane. X-Plane, as it turns out, can be made to closely simulate the conditions found on Mars. Unfortunately, as was also found, flight on Mars is possible but difficult. To achieve flight on Mars, you need to be going fast. The article states that a speed of mach 1 is required merely to achieve flight. Problem is, once you achieve flight, the inertia makes it nearly impossible to change course.

Venus: Venus is interesting. The atmosphere on Venus is 60 times denser than Earth's atmosphere. You could easily achieve flight at incredibly low speeds (a Cessna 172 Skyhawk, the aircraft the article is based around, could achieve flight at running speed). Problem is, the air on Venus is hot enough to melt lead. You can always get around this by flying in Venus's upper atmosphere. The upper atmosphere is rather earth-like and would be quite easy to fly a plane in. Only, you'd have to ensure no metal is exposed as sulfuric acid in the upper atmosphere introduces the threat of corrosion.

Jupiter: Flight on Jupiter is unrealistic. Jupiter's gravity is much too strong. The power required to maintain flight is about 3x that of Earth making flight there highly unrealistic.

Saturn: Weaker gravity and slightly denser atmosphere than Jupiter means an aircraft might fair better but ultimately would succumb to cold or high winds.

Uranus: Flight on Uranus could be sustained slightly longer but ultimately the aircraft would still succumb to the conditions found there.

Neptune: The temperature and turbulence make it impossible to achieve flight on Neptune. It's assumed your aircraft would quickly break apart in the atmosphere.

Titan: Titan is perhaps the best plan to attempt flight on. To quote the article:

"When it comes to flying, Titan might be better than Earth. Its atmosphere is thick but its gravity is light, giving it a surface pressure only 50% higher than Earth’s with air four times as dense. Its gravity-lower than that of the Moon-means that flying is easy."

Flight on Titan IS easy. A human could theoretically achieve flight with a wingsuit and mere muscle power. The problem is, Titan is cold, 72 Kelvin cold. Flight would require some major heating modifications but, barring the heat factor, Titan is the absolute best place to attempt flight in our solar system. It's even better than Earth. As an interesting note, Titan, thus far, has actually been too cold for even unmanned probes to explore. Again, quoting the article:

The batteries would help to keep themselves warm for a little while, but eventually the craft would run out of heat and crash. The Huygens probe, which descended with batteries nearly drained (taking fascinating pictures as it fell), succumbed to the cold after only a few hours on the surface. It had enough time to send back a single photo after landing—the only one we have from the surface of a body beyond Mars.

Earth: Earth's conditions are quite optimal for flying. Earth's gravity is 9.78 m/s². As a comparison, Jupiter's gravity is 24.79 m/s² and Titan's gravity is 1.352 m/s². Earth's atmosphere is, at sea level, 1 standard atmosphere or 101.3 kPa or 14.7 psi compared to Mars's average which is about 0.006 standard atmosphere or 600 Pa or 0.087 psi and Venus's average which is about 9.2 mPa or 1,330 psi. Takeoff speed for our Cessna 172 Skyhawk is 64 KIAS (Knots Indicated Air Speed) and the best rate of climb is 73 KIAS. Normal cruise speed in a Cessna 172 Skyhawk is 122 knots (140mph, 226 km/h). As a comparison, flight on mars would require speeds over Mach 1 which translates to 768 mph or 1,236 kph.

To summarize:

  • Sun: Instant vaporization.
  • Mars: Atmosphere's too thin to fly below mach 1, above mach 1 you essentially can't steer.
  • Venus's Lower Atmosphere: Flight is possible but the air's hot as lead. You'd melt.
  • Venus's Upper Atmosphere: Flight is possible but corrosion is a factor due to sulfuric acid so no exposed metal.
  • Jupiter: High gravity makes flight extremely unrealistic.
  • Saturn: Flight's possible but your aircraft might ultimately succumb to the cold and weather conditions.
  • Uranus: Same as Saturn but you MIGHT last a bit longer.
  • Neptune: Your aircraft would break apart quickly from the extreme turbulence.
  • Titan: Flight could be achieved with artificial wings and mere muscle power. Unfortunately, Titan's cold. To quote the XKCD article:

If humans put on artificial wings to fly, we might become Titan versions of the Icarus story—our wings could freeze, fall apart, and send us tumbling to our deaths.

  • Earth: We know flight on Earth works due to firsthand knowledge. We don't have the most optimal conditions in our solar system, but the conditions here are still great for all types of manned aircraft.
  • Anywhere Else: No atmosphere, so you would crash ballisticly.

As a small note:

Titan is the absolute best environment for flight using a conventional aircraft if you don't factor in the cold. I imagine it would be much easier and less costly to attempt flight in Venus's upper atmosphere by protecting all exposed metal from corrosion than it would be to make major modifications to a conventional aircraft so that it and its pilot can withstand the extreme cold found on Titan.

Another Small Note: Mach 1 is measured relative to earth so 340.29 m / s. The speed of sound on Mars is different. The speed of sound is 226 m/s.

Are there moonmoons orbiting other moons ?

What do you call a moon that is in orbit around another moon ? Image Credit: NASA / Sean Smith

While we currently don't know of any real world examples of moons orbiting other moons, two scientists - Juna Kollmeier and Sean Raymond - recently determined that the idea is plausible.

By using equations designed to calculate the tidal effects of planets on their moons, the pair worked out that a moon can potentially exist around another moon so long as the host moon is sufficiently large, the submoon is sufficiently small and a wide enough gap exists between them.

On this basis, it is theoretically feasible for some of the moons in our solar system to have smaller moons of their own, including Saturn's moon Titan, Jupiter's moon Callisto and even our own moon.

But if we did discover such a moon, what would we call it ?

Some of the terms that have been used to describe such a theoretical body include submoon, moonito, grandmoon, moonette, moooon and the increasingly popular moonmoon.

And what would you call a moon orbiting a moon that is itself orbiting another moon ?

Until such a thing is discovered however, it is unlikely that an official term will ever be needed.

Similar stories based on this topic:

The Fission Theory Or the “Budding” of the Moon

W hen we think of the name ‘Darwin,’ we immediately think of Charles Darwin, one of the greatest scientists of all time, author of that On the Origin of Species that occupies a place of absolute relevance in the world scientific literature. But the history of science also includes another Darwin, certainly much less famous than Charles. This is George Howard, the fifth son of Charles and Emma Darwin.

Born in 1845, George H. Darwin b e came an astronomer of solid and recognized preparation, so much so that in 1892 he won the prestigious gold medal of the Royal Astronomical Society, of which he was later even president. In 1883 he obtained the chair of astronomy and experimental philosophy at the University of Cambridge, founded in 1704 by Thomas Plume and considered one of the two most important chairs of astronomy in that university (the other is the one established by the astronomer Thomas Lowndes in 1749).

George Darwin was a great tide expert. In fact, all his scientific production mostly revolved around tides, the phenomena connected to them, and the theories that explained them. We could almost say that he was obsessed with the tides. Also, tides are linked to a hypothesis on the formation of the Moon supported by the astronomer Darwin, known as the fission theory. The following text reports the salient passages that describe this conjecture, taken from a book that George Darwin published in 1899, entitled, needless to say, The Tides (pp. 281–284):

… I say that if a planet, such as the earth, made each rotation in three hours, it would very nearly fly to pieces. The attraction of gravity would be barely strong enough to hold it together, just as the cohesive strength of iron is insufficient to hold a fly-wheel together if it is spun too fast. There is, of course, an important distinction between the case of the ruptured fly-wheel and the supposed break-up of the earth for when a fly-wheel breaks, the pieces are hurled apart as soon as the force of cohesion fails, whereas when a planet breaks up through too rapid rotation, gravity must continue to hold the pieces together after they have ceased to form parts of a single body.

Hence we have grounds for conjecturing that the moon is composed of fragments of the primitive planet which we now call the earth, which detached themselves when the planet spun very swiftly, and afterwards became consolidated. …

Is there, then, any other cause which might cooperate with rapid rotation in producing rupture? I think there is such a cause, and, although we are here dealing with guesswork, I will hazard the suggestion.

The primitive planet, before the birth of the moon, was rotating rapidly with reference to the sun, and it must therefore have been agitated by solar tides. … there is a general dynamical law which enables us to foresee the magnitude of the oscillations of a system under the action of external forces. That law depended on the natural or free period of the oscillation of the system when disturbed and left to itself, free from the intervention of external forces. We saw that the more nearly the periodic forces were timed to agree with the free period, the greater was the amplitude of the oscillations of the system. Now it is easy to calculate the natural or free period of the oscillation of a homogeneous liquid globe of the same density as the earth, namely, five and a half times as heavy as water the period is found to be 1 hour 34 minutes. The heterogeneity of the earth introduces a complication of which we cannot take account, but it seems likely that the period would be from 1½ to 2 hours. The period of the solar semidiurnal tide is half a day, and if the day were from 3 to 4 of our present hours the forced period of the tide would be in close agreement with the free period of oscillation.

May we not then conjecture that as the rotation of the primitive earth was gradually reduced by solar tidal friction, the period of the solar tide was brought into closer and closer agreement with the free period, and that consequently the solar tide increased more and more in height? In this case the oscillation might at length become so violent that, in cooperation with the rapid rotation, it shook the planet to pieces, and that huge fragments were detached which ultimately became our moon.

There is nothing to tell us whether this theory affords the true explanation of the birth of the moon, and I say that it is only a wild speculation, incapable of verification.

In summary, Darwin believed that the primordial Earth had not yet solidified at the time when its rotation period was between three and four hours. Then, under the action of the centrifugal force imposed by the fast rotation, the sloshing of its fluid interior produced a periodic oscillation. Taking into account the average density of our planet (5.5 grams per cubic centimeter), Darwin calculated an oscillation period of between 1.5 and 2 hours. This oscillation, not sufficient to shatter the Earth, had to be integrated with another periodic oscillation — the one created twice a day by the solar tides.

However weak in themselves, the solar tides could trigger a multiplier effect, once the progressive slowdown of the Earth’s rotation, generated by those same tides with their friction, produced a period of oscillation of the “fluid Earth” exactly coinciding with that of the solar tides. It is the same principle that prevents an army from marching in unison on a bridge. The steps of each soldier create an insignificant disturbance, but, summed up with all the others, they produce sound waves which, being in phase with each other, add up in amplitude and can generate vibrations so powerful to induce a bridge to collapse.

In the process imagined by George Darwin, depicted in the vintage illustration reproduced below, the fluid Earth gradually generated a bulge. The planet, no longer similar to a sphere, became a sort of planetary egg. Under the combined action of free swing and solar tides, the egg gradually transformed into a “pear,” and the pear finally produced, like a sort of bud, the Moon, or rather the large orbiting fragments which — based on this conjecture — thickened and solidified up to form the Moon. George Darwin believed he had even identified in the Pacific Ocean the place on Earth from which matter was torn and then ended up in our satellite. Therefore, the great depression filled by the water of this ocean would be the planetary “wound,” which has remained in perpetual memory of the shattering of the primordial Earth.

George Darwin’s main merit was that he sought to base his theory of the formation of the Moon on a scientifically sound basis. It is undoubtedly a fact that the Earth of the origins rotated faster than today’s Earth, just as it is certain that the Moon is still moving away and that the tides, with their friction, generate equatorial bulges and affect the period of rotation. Everything else, however, as Darwin himself acknowledged, is highly speculative.

Today very few credit Darwin’s theory of lunar formation, also because it has severe theoretical gaps, related in particular to the issues of conservation of angular momentum and the minimum distance at which a satellite could have existed without being shattered by Earth’s gravity. However, the fission theory also has its strengths. For example, it explains very well the lower average density of the Moon compared to Earth. If the Moon was born from materials torn from the Earth’s mantle, less dense than the iron core of the planet, it follows that the average density of the Moon must be more similar, as it is, to that of the Earth’s mantle that not to that of the core.

As it may be, Darwin’s hypothesis today is considered more a curiosity from the history of astronomy, while most scientists reserve their favor to other conjectures. In particular, the theory currently most credited on lunar formation is that of the so-called “giant impact,” that is, the supposed clash between the primordial Earth and a body the size of Mars, which astronomers named Theia. Following the crash with Theia, a cloud of debris orbiting around what remained of our planet would have gradually condensed to form the Moon. Unfortunately, no one was there to witness what really happened four and a half billion years ago. It is, in fact, the age of the Moon that is obtained from the analysis of the moon rocks brought (back) to Earth by the astronauts of the Apollo missions.


Moons are natural objects that orbit planets. Scientists usually refer to them as planetary satellites (human-made satellites are sometimes called artificial moons). There are about 170 moons in our Solar System. Most of them are in orbit around the gas giants Jupiter and Saturn. Small planets tend to have few moons: Mars has two, Earth has one, while Venus and Mercury do not have any.

Earth’s Moon is unusually large compared with the planet. Most moons are dwarfed by their nearby planet. However, some of the moons of Jupiter and Saturn are much larger than our Moon. Ganymede and Titan are bigger than the planet Mercury.

Their small size means that almost all moons are unable to hold onto atmospheres. Their weak gravity allows the gases to escape into space. The odd one out is Titan, the largest moon in Saturn’s system. Titan has a thicker, denser atmosphere than Earth. Nitrogen is the main gas in both of their atmospheres, but Titan has no oxygen and no life. Its surface is hidden by an orange haze. Some scientists think of Titan as a primitive Earth in deep freeze.

Some satellites, including our Moon, are thought to have been born during massive collisions, early in the history of the Solar System. The Moon formed from the cloud of debris that was left in orbit around the Earth. Others may have grown from a cloud of gas and dust that surrounded the giant planets when they were pulling in material. Most of the smaller moons are comets or asteroids that were captured when they passed too close to a large planet.

Wobbly Planets Could Reveal Earth-like Moons

Moons outside our Solar System with the potential to support life have just become much easier to detect, thanks to research by an astronomer at University College London (UCL).

David Kipping has found that such moons can be revealed by looking at wobbles in the velocity of the planets they orbit. His calculations, which appear in the Monthly Notices of the Royal Astronomical Society December 11, not only allow us to confirm if a planet has a satellite but to calculate its mass and distance from its host planet &ndash factors that determine the likely habitability of a moon.

Out of the 300+ exoplanets (planets outside our Solar System) currently known, almost 30 are in the habitable zone of their host star but all of these planets are uninhabitable gas giants. The search for moons in orbit around these planets is important in our search for alien life as they too will be in the habitable zone but are more likely to be rocky and Earth-like, with the potential to harbour life.

&ldquoUntil now astronomers have only looked at the changes in the position of a planet as it orbits its star. This has made it difficult to confirm the presence of a moon as these changes can be caused by other phenomena, such as a smaller planet,&rdquo said David Kipping. &ldquoBy adopting this new method and looking at variations in a planet&rsquos position and velocity each time it passes in front of its star, we gain far more reliable information and have the ability to detect an Earth-mass moon around a Neptune-mass gas planet.&rdquo

The appearance of wobbles in a planet&rsquos position and velocity are caused by the planet and its moon orbiting a common centre of gravity. While the old method of looking at the wobbles in position allowed astronomers to search for moons, it did not allow them to determine either their mass or their distance from the planet.

Professor Keith Mason, Chief Executive of the Science and Technology Facilities Council, said, &ldquoIt&rsquos very exciting that we can now gather so much information about distant moons as well as distant planets. If some of these gas giants found outside our Solar System have moons, like Jupiter and Saturn, there&rsquos a real possibility that some of them could be Earth-like.&rdquo

Kippings work is funded by the UK&rsquos Science and Technology Facilities Council (STFC).

Terrestrial Bodies in the Solar System

The homework for today focuses on an asteroid even though we won't talk about asteroids until next week .

  • terrestrial bodies like the Earth
  • gas giants like Jupiter
  • everything else (asteroids, comets, dust, etc.)

Today, we'll take a look at the major terrestrial bodies tomorrow, we'll turn our attention to the gas giants. Terrestrial bodies are simply those with solid surfaces on which one could stand. The Earth is a nice terrestrial planet.

Gallery of the major terrestrial bodies

Below is a "family portrait" of the largest terrestrial bodies in the Solar System. All are viewed from the same distance (about 53,000 km) so that the pictures reflect their relative sizes correctly.

First, we have the really big ones: clockwise from upper left, Earth, Mercury, Mars and Venus.

Next, the four large moons of Jupiter: clockwise from upper left, Io, Europa, Callisto, Ganymede.

Finally, three more large moons: Titan and Triton in the upper row, and, at lower left, the Earth's Moon. Last of all, at lower right, is Ceres, the largest asteroid. It doesn't really belong here, but I include it so that you can see how it (and smaller asteroids) compare in size to the big boys.

Ultra-hostile: Venus

Venus is NOT a good place to crash-land your spaceship. Why? Well, the main problem is its very, very thick atmosphere. The picture below shows it with (on left) and without (on right) its layers of clouds. The coloring of the surface isn't accurate, by the way: it isn't really bright yellow.

  • surface pressure: 90 Earth atmospheres
  • composition: 96% carbon dioxide, 3.5% nitrogen, plus water vapor, sulfuric acid, hydrochloric acid, hydrofluoric acid
  • temperature at the surface: about 740 Celsius

Why is Venus so hot? Well, it is closer to the Sun than the Earth is.

No, the reason that Venus is so much hotter than the Earth is that its thick atmosphere traps much of the heat: it transmits much of the visible and near-infrared light from the Sun down to the surface, but then absorbs the mid-infrared radiation emitted by the surface. It acts like the translucent panels of glass in a greenhouse:

Note that the surface temperature of Venus, over 700 Celsius, is hot enough to melt lead, tin, zinc, and other metals. It would be very difficult to keep any spaceship cool enough for its electronics to function for very long. As a matter of fact, the only spacecraft to soft-land on Venus, the Soviet Venera 9, 10 and 13, returned data for only a few minutes before succumbing to the conditions.

See other pictures from the surface of Venus at Ted's Venera 13 page

Oh, one more thing: Venus has a relatively young surface, due to ongoing volcanic activity. Just one more thing to brighten your day (hour? minute?) on the surface.

Hostile: the Galilean Moons of Jupiter

Jupiter has four large moons, first observed by Galileo when he turned his telescope on the planet.

What's that little speck to the left of Io? The irregular Jovian moon Amalthea.

Because they formed far from the Sun, these bodies have little refractory material (heavy elements with high melting and vaporization points, such as iron, nickel, silver, gold, platinum, etc.) they are instead made up mostly of lighter substances with lower vaporization temperatures: oxygen, carbon, silicon, hydrogen-rich compounds. The basic idea is that the Solar System was originally a cloud of mostly gas and dust, with small amounts of refractory materials. Close to the Sun, the heat of the Sun vaporized dust particles and prevented the resulting gas from condensing all that was left was the residual refractory materials.

The overall density of these moons is therefore lower than that of terrestrial bodies in the inner reaches of the Solar System:

The Galilean moons have long served as outposts for scientists and explorers in science fiction. In Arthur C. Clarke's book (and movie) 2010, for example, after the monoliths have turned Jupiter into a low-luminosity star by waving their magic wands, they tell humanity that "all these worlds (the Galilean moons) are yours, except Europa." Europa, in these books, has a native ecosystem the monoliths wish to protect. So, why have I classified them as hostile to stranded spacemen?

A minor issue is the tectonic activity on Io. Its surface is covered with sulfer-rich material emitted from many calderas and vents.

Each time the Galileo spacecraft flew past Io, it saw active eruptions and "lava" flows from several spots on the surface the visible and near-IR image below, taken in 1997, shows several hot spots: the volcano Pillan is the brightest source, with Pele just to its lower left.

But the REAL reason that you would not want to be stranded on any of the Galilean moons is the deadly radiation. Jupiter has a very, very strong magnetic field which stretches far beyond the planet's cloudtops. The Cassini spacecraft detected particles trapped in these strong magnetic fields when it passed through the Jovian system in 2001: the picture below shows regions of "Energetic Neutral Atoms (ENA)" in red, with additional concentrations of materials near the orbits of Io (green) and Europa (blue).

These energetic particles would pose a serious health risk to astronauts. The only way to avoid them would be to build shields with thick layers of matter to block the high-energy particles. Probably the easiest solution would be to dig bunkers and live underground but this would take a good deal of time and heavy machinery.

Neutral: Mercury, the Moon, and Triton (?)

From our everyday point of view, we would not survive long if suddenly transported to any of these worlds: none of them has much of an atmosphere, for one thing. Mercury and the Moon are bare, dense balls of rock (though Mercury has a large core of iron). This picture of Mercury, taken by the Mariner 10 spacecraft in 1974 (or 1975), could stand for either body.

Now, Mercury is very close to the Sun, orbiting at roughly 0.39 AU. The temperatures on the surface reach over 300 Celsius during the long day (which lasts about 176 Earth days), and plunge to less than -160 Celsius during the equally long night. Since the Moon also lacks an atmosphere to transfer energy around the surface, it, too, suffers extreme temperature changes from day to night since it it farther from the Sun, their range is somewhat smaller: from about -170 Celsius to about 130 Celsius. Large temperature ranges are difficult to endure, it's true.

Moreover, the very tiptops of these same craters' rims would always, or nearly always, see the Sun. It is possible, then, that the polar regions of these bodies might turn out to be pretty nice places: one would have a supply of solar energy and a store of water (from which oxygen could be extracted). It is much easier to build and maintain a comfortable human environment in a cold vacuum (as on the Moon and Mercury) than in a hot, dense atmosphere (as on Venus).

The last body in this category is Triton, the largest moon of Neptune.

It has a very thin atmosphere, roughly 100,000 times less dense than Earth's, composed mostly of nitrogen. Since Triton, like Neptune, is very far from the Sun -- about 30 AU -- it is very cold: the surface temperature is roughly -238 Celsius. So why do I place this into the "neutral" category? As mentioned above, it isn't a very difficult engineering task to keep a habitat warm in a vaccuum. Triton lacks the abundant solar energy of the inner Solar System, but its surface is probably composed of icy materials which would easily yield useful chemicals such as hydrogen, oxygen, carbon and nitrogen.

Friendly: the Earth, Mars and Titan (?)

You all know about the Earth, so I won't say anything else about it now.

For centuries, Mars has been seen as the most likely abode for life in the Solar System outside the Earth. Telescopes showed clearly that it had an atmosphere with occasional clouds and dust storms, and polar "ice" caps which grew and shrank with the Martian seasons. Consider these pictures, taken with small telescopes by Donald Parker, an amateur astronomer who does great work monitoring the planet.

Since Mars is a bit farther from the Sun than the Earth, the surface temperatures a bit lower than ours: the air temperature near the equator ranges from slightly above freezing (0 Celsius) to less than -100 Celsius. It doesn't range as widely as the temperature on the Moon because the Martian atmosphere carries heat across the planet. That atmosphere is not thick enough to sustain human life -- only about one percent the density of the Earth's -- and, since it's mostly carbon dioxide, you wouldn't want to breathe it, anyway.

The polar caps contain a mixture of frozen carbon dioxide and water ice, which would be very handy for stranded humans.

Finally, let's look at Titan, the largest moon of Saturn. Until recently, we were unable to peer beneath its thick cover of clouds, as in this image from Voyager 2.

We knew that Titan had a thick atmosphere, about 50 percent denser than Earth's, made up mostly of nitrogen, with small amounts of methane and other hydrocarbons. We also knew that Titan was cold, with a surface temperature of about -180 Celsius. Some scientists speculated that the surface might have an exotic chemistry, with liquid oceans of methane and ethane not a bad place, perhaps, for complicated molecules to grow .

In early 2005, the Cassini spacecraft swept into the Saturn system. Its infrared camera was able to penetrate the cloud layers and see Titan's surface features at last:

Kevin Dawson put together a poster which combines some of the first images to be released from the Huygens probe. Click on the image below for the giant, full-size version.

It looks like some of the surface really is covered with liquids .

Titan isn't really "friendly" to human life, but it might be a good place to look for other sorts of life: over the past four billion years, all sorts of interesting chemical reactions may have been occuring in its dense atmosphere and oceans.

For more information

  • The Nine Planets is THE place to go for more information on bodies in the Solar System.
  • The Celestia software package made some of the pretty pictures in today's lecture.
  • Ted Stryk has collected nice images of many solar system bodies, and brought out extra details in some.
  • JPL's Planetary Photojournal provides great pictures from its spacecraft, plus short descriptions written by scientists.
  • Comparison of magnetic fields of the giant planets
  • A small collection of resources on the question of water in lunar polar regions
  • A great web site run by space enthusiasts collects information on the Huygens mission to Titan.

Copyright © Michael Richmond. This work is licensed under a Creative Commons License.

Earth has 3 moons?!

Have they/ will they consolidate over time? How heavy are these clouds and what does this mean for gravitational pulls?

Edit: I am kinda sad that out astronomy community is not wondering about this: how did we ignore the anomalies that must have definitely showed up while verifying the applicability of Newton’s or Einstein’s gravitational laws? Even if the anomalies was minute fractions? Where’s the human curiosity? Or doubt? :(

That's actually really cool! When I first read it I though that it would be some stupid conspiracy theory like the flat earth society or something

That’s ridiculous. Everyone knows the earth is actually square.

There are also other objects (I find the term moon not fitting) From Wikipedia :

469219 Kamoʻoalewa, an asteroid discovered on 27 April 2016, is possibly the most stable quasi-satellite of Earth.[4] As it orbits the Sun, 469219 Kamoʻoalewa appears to circle around Earth as well. It is too distant to be a true satellite of Earth, but is the best and most stable example of a quasi-satellite, a type of near-Earth object. They appear to orbit a point other than Earth itself, such as the orbital path of the NEO asteroid 3753 Cruithne. Earth trojans, such as 2010 TK7, are NEOs that orbit the Sun (not Earth) on the same orbital path as Earth, and appear to lead or follow Earth along the same orbital path.