What do we know about exomoons?

Artist’s depiction of an exomoon candidate

Unfortunately, this question has an easy answer: not much. So far, no exoplanet has been confirmed to have a moon, even though scientists are detecting planets the size of the Jovians. Even though nothing has been confirmed, however, there have been some interesting potential discoveries. We say potential because again, the systems are so far away it is hard to confirm anything.

One astronomer from the University of Padua in Italy, Cecilia Lazzoni, claims she found two giant exomoons. In both cases, the planets are about 11 to 13 times as large as Jupiter, and their moons are around Jupiter size. The question is if these systems can even be understood as planets and their orbiting moons. Some say these planets could be classified as brown dwarfs, objects that can only complete half of the proton-proton chain and thus don’t achieve star status. Brown dwarfs are normally classified as 13 times as large as Jupiter, but the definition is completely clear. If the object is a brown dwarf, then the moon could actually be a planet. Another explanation for these systems could be calling them binary planets, similar to the idea of binary stars.

Other researchers from Columbia University claim to have evidence of an exomoon that is around Neptune size, orbiting around a planet several times as large as Jupiter.

This kind of discovery is exciting because, even if they aren’t called exomoons but end up being planets, they force us to expand how we classify and think of extrasolar systems. Additionally, similar to how astronomers consider Europa and other Jovian moons as possibilities for containing life, some consider exomoons as candidates for life outside our solar system. Dr. Phil Sutton from the University of Lincoln said,

“These moons can be internally heated by the gravitational pull of the planet they orbit, which can lead to them having liquid water well outside the normal narrow habitable zone for planets that we are currently trying to find Earth-like planets in…I believe that if we can find them, moons offer a more promising avenue to finding extra-terrestrial life.”

Exactly like our Jovian moons! So although exomoons might offer possibilities outside of what we know about our own solar system, we can still apply the knowledge we find in our solar neighborhood to other systems far, far away.

Sources: Have Astronomers Detected Exomoons at Last?, Exomoons May Be the Best Place to Search for Life

8 thoughts on “What do we know about exomoons?

  1. This is fascinating! Thinking about why finding exomoons is challenging is an interesting exercise. I think it’s most likely difficult because 1. Moons are smaller than the planets they orbit–and we’re bad at finding small things. For example, the vast majority of exoplanets we’ve discovered are like the megaJupiter you discuss in your blog post, since our instruments are just largely not sensitive enough to track the effects of smaller planets. 2. That planets don’t emit as much light as stars. One of the textbook ways to discover a planet is when it’s in transit around a star, as it will cause dimming of said star. For exomoons, this is nearly impossible, since the amount of light they reflect from a star would be so small in the first place that detecting dimming would be extremely difficult (not to mention there are other possible explanations, like differing albedo on different sides of the planet). The search for exmoons is both intriguing and challenging, and your blog post is a good introduction to their importance.

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    1. Thank you! Because the methods we use to detect exoplanets won’t work for exomoons, or won’t work as well, I wonder if we will have to find new ways of detecting extrasolar bodies to prove their existence. We may not get confirmation for exomoons until we have better instruments and more development in technique. This could definitely be an area in astronomy that could expand in the next couple of decades, just as the search for exoplanets has expanded so much since the 1990s.

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      1. The newest way to detect exoplanets is a technique called microlensing. It will be sensitive to planet sizes that are not currently easy to find. It may be worth investigating if microlensing can detect exomoons.

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      2. That sounds really fascinating – I didn’t know there were other ways to detect exoplanets outside of the transit method and using radial velocity. Is microlensing related to gravitational lensing?

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      3. Yes, microlensing uses gravitational lensing to look for exoplanets. This is beyond my expertise but there are some in the astrophysics department who study microlensing.

        Also, there are about five ways to detect exoplanets: pulsar timing, direct imaging, transit, radial velocity, and microlensing. There may be others but these are the five I know about.

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  2. I believe that there are countless number of exomoons out there. They are probably undetectable because they are so small. It hard for scientists to detect even terrestrial planets because of their much smaller size compared to the star that they orbit. This was an interesting read and I completely agree that we can apply the knowledge learnt from studying our solar system to solar systems elsewhere!

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    1. Thank you! I totally agree that there must be a large number of exomoons out there, just like exoplanets. It will be interesting to see how the field of detecting extrasolar objects will expand. I think as we are able to detect more terrestrial planets that we’ll be able to confirm exomoons, especially since some of them may be the size of Jovian planets.

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