There are eight planets and more than 200 moons in the Solar System. The majority of the moons have no chance of being hospitable, but some of them are candidates for life. Is it the same in other solar systems? The Moons in our Solar System show a wide range of colors. There are moons where the ocean is covered in ice. There is a moon with bodies of liquid. There is a moon that experiences volcanic activity. accretion formed some moons. Earth was formed from a collision between planets. Some moons may have been captured by asteroids. Some moons may be captured. Some of the planets and moons are so small they are barely moons at all. We can see exomoons on exoplanets, which is why the search is focused on them. Even though there are no exomoons, exoplanets must have them. The authors of a new paper say that they have no reason to think that moon formation is different from the Solar System. There may be more exomoons than there are exoplanets. Is there a way to understand what exomoons are looking for?
The authors think so. The study will be published in the Monthly Notices of the Royal Astronomical Society. Vera Dobos is an astronomer at the Kapteyn Astronomical Institute.
What we know about the habitability of exoplanets isdwarfed by what we don't know. Studies like this are new to understanding exomoon habitability. We haven't found our first exomoon yet, but we will soon. It looked like the astronomer might have found one in 2018, but now it's not certain. The science community is preparing for exomoon discoveries thanks to studies like this one.
We may face a lot of exomoon discoveries when we find them. Basic knowledge about exomoons by the first detections will help in selecting further observation targets.
Even though no exomoons are known today, their potential habitability is an interesting and important topic. Their study gives us a target list to maximize our chances of finding exomoons.
Large rocky exomoons can hold onto their atmospheres. The authors looked at the heat available to exomoons from the stars in their solar system. They didn't pay attention to radiogenic heating, which could provide enough heat to maintain a ocean.
The researchers excluded exoplanets based on the following criteria.
They ended up with 4140 known exoplanets. Each of those planets had 100,000 moons. Habitability probability is the ratio of test moons compared to the number of simulations.
The team identified 234 known exoplanets where the moons had a 1% habitability probability. Habitability probabilities greater than 50% were found in 17 of the planets.
The sizes of the markers represent the sizes of the planets, as shown in the legend. There are different shades of green lines that show conservative and optimistic zones for the exoplanets. The size of the markers in the plot is scaled to the actual size of the planets in the legend. The image is from Dobos et al.
The study showed that most of the moons are in the right place. Several massive known exoplanets have a high habitability probability. The study showed that planets with high tidal-heating rates were more likely to host moons outside their stars.
The host stars change these results. The Extrasolar Planets Encyclopedia doesn't list the type of star for over half of the exoplanets it contains. The current evolutionary state of the stars is not listed. Some of the stars are not good at habitability. Even if the simulations resulted in higher habitability probabilities, the star might deny that. Suppose a star has extreme flaring and radiation. A planet with a protective magnetosphere could be used to protect an exomoon.
Habitability of exomoons is determined by many other factors. When planets migrate, they can lose their moons. Depending on the physical and orbital parameters of the planet and the moon, some moons can survive. The scope of the paper excludes those factors.
The table of exoplanets that have high probabilities of hosting potentially habitable moons was found by this work. Readers might recognize some of the exoplanets with the highest probabilities.
The planet with the highest probability of being a hospitable environment is called Kepler-459 b. It is a sub-Jupiter that is circling a star like our Sun every 856 days. The exomoon habitability probability is 70%. The sub-Jupiter is about the same mass as the Sun and takes 1320 days to travel around it. The exomoon habitability probability is 69%.
HD 7199 b is third on the list. It is a gas giant that circles its star every 1.7 years. HD 7199 has an exomoon habitability probability of 64%.
The first rocky super-Earth found around a Sun-like star is further down the list. It's sometimes called Earth's cousin, or Earth 2.0. The exomoon habitability probability is 60%. Then there's the planet. One of the most promising habitable exoplanet candidates is called Kepler-62f, because it is very stable and long-lived. It has an exomoon habitability probability of 53%.
Proxima Centauri c is on the list. It gets attention because it is so close to us. It is near the bottom of the list with an exomoon habitability probability of 14.
Many exoplanets have very long orbital periods. It's difficult to look for exomoons on the planets. The shorter orbital periods of others, like the Kepler-62f, are only a couple hundred days. It is an intriguing target in the hunt for exomoons.
Large gas giants are some of the exoplanets on the list. They could host a lot of exomoons, like the small planets in our Solar System. Galileo found four large moons when he pointed his telescope at Jupiter. His discovery was a big deal. Maybe our first exomoon detection will be similar, and we will find our first four all at once.
We will have the technology and methodology to find exomoons more quickly. We may soon know of thousands or even tens of thousands of exomoons. When that happens, this study might help the astronomer decide where to look.
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