Of the roughly 6,000 exoplanets we’ve discovered, a significant number are in the apparent habitable zones of their stars.
Most are giant planets; either gas giants like Jupiter and Saturn, or ice giants like Uranus and Neptune. Could some of those have habitable exomoons?
No life could exist on our Solar System’s giant planets. However, some of their moons have become prime targets in the search for life.
It leads to a natural question: Could giant exoplanets in habitable zones around other stars have habitable moons?
Astronomers have detected only tantalizing hints of exomoons, even though their existence is virtually guaranteed. Theory shows that moon formation is a natural process.
Finding exoplanets is difficult, even though we’ve become used to it, and finding their moons is even more difficult.
Researchers from Hungary and the Netherlands wanted to study how exomoons might form around distant, giant planets to gain insight into their existence.
Their research is titled “Grand Theft Moons: Formation of habitable moons around giant planets,” and it will be published in Astronomy and Astrophysics. The lead author is Zoltán Dencs from the HUN-REN Research Centre for Astronomy and Earth Sciences.
“We aim to study moon formation around giant planets in a phase similar to the final assembly of planet formation,” the authors write.
“We search for conditions for forming the largest moons with the highest possibility in circumplanetary disks, and investigate whether the resulting moons can be habitable.”
It starts with circumplanetary disks, the rotating collection of material that remains after a planet forms. The researchers used simulations to determine what fraction of that material can successfully form moons. In this case, the researchers focused on the most massive moons.
“We determined the fraction of the circumplanetary disk’s mass converted into moons using numerical N-body simulations where moon embryos grow via embryo−satellitesimal collisions,” the researchers write.
They examined the disks around giant planets where 100 lunar embryos interact with 1000 satellitesimals. The planets were 461 known giant exoplanets from an exoplanet database.
A habitable zone for planets depends on the stellar irradiation coming from the star. With enough energy, liquid water can persist on a planet’s surface, given the right atmospheric conditions and other factors.
For moons, the formula is a bit different. In our Solar System, icy moons like Europa and Enceladus likely have liquid water under a frozen cap, but the heat comes from tidal flexing. The researchers included that heat in their simulations.
“To determine the habitability of the synthetic moons, we calculated the stellar irradiation and tidal heating flux on these moons based on their orbital and physical parameters,” the authors write.
“The global energy flux on the moons can be significantly influenced by tidal heating, which comes from the tidal energy dissipation of the planet−moon interactions,” they explain.
As our solar system shows, tidal heating becomes more significant the further a moon is from its star.
The team’s simulations involved circumplanetary disks in the final phase of moon formation. For simplicity, they involved rocky bodies only and gas-free disks. “The disks consist of moon embryos embedded in a swarm of satellitesimals, and the only force considered in the calculation is gravity,” they write.
All objects—the star, the planet, the embryos, and the satellitesimals—interact gravitationally. The simulations allowed embryo-embryo or embryo-satellitesimal collisions, but not collisions between satellitesimals. They also included hot and cold disks, and other factors like the eccentricity and inclination of embryos and satellitesimals.
As bodies in the simulation reacted with one another, there were four different results.
In the first result, the objects combined and added their mass together. In the second, the planet accretes the object. In the third, the body is accreted by the star. In the fourth, the body is ejected from the system. Only the first result forms exomoons.
The simulation included two timescales: the number of planetary orbits around the star and the number of orbits for the proto-satellites in the circumplanetary disk. The first is stellar-centred (SC) and the second is planet-centred (PC).
The first question regards mass loss. Do the disks retain enough mass to form habitable moons? The researchers discovered that the entire circumplanetary disk loses mass over time. As some embryos become more massive, their perturbations dissipate mass from the disk, shrinking the overall embryo mass.
The most significant mass loss is when the exomoons are in cold disks within 1 AU of the star, as panel A shows above. In that situation, the disk loses between 30% and 40% of its mass. Panel B shows that while embryos lose mass in the planet-centred simulation, it’s not as extreme. They retain more than 90% of their initial mass.
The simulations provide much more detail, but the results show that exomoons should form and remain in circumplanetary disks around giant planets. This is despite mass loss, ejections, and embryos absorbed by the star or the planet.
As the stellar distance increases, the number of moons rises. However, their initial masses are smaller. As the mass of the exomoons rises, more of them are lost to stellar theft. “Due to these two factors, the highest moon formation efficiency is observed for the planet orbiting at two au stellar distance,” the authors write.
Habitability is a separate question, and the simulations had some interesting results.
Beyond about one au, tidal heating becomes the primary heating source for habitable exomoons. The simulations also showed that beyond two au, the number of habitable exomoons decreases dramatically because the habitable zone shrinks. “The optimal distance for habitability is between 1−2 au stellar distances,” the researchers explain.
They also found that the number of exomoons increases as stellar distance increases. However, their masses are too small, making them uninhabitable.
“We examined the habitability of putative Earth analog moons around 461 known giant exoplanets, selected by their mass,” the researchers write in their conclusion. “Our simulations show that moons with masses between Mars and Earth could form around planets with masses about 10 times that of Jupiter, and many of these moons could be potentially habitable at 1−2 au stellar distances.”
The study shows that when searching for habitability, we should expand our scope to include more than just rocky, habitable zone exoplanets. We should begin searching for habitable exomoons at greater distances from their stars. “These locations provide suitable targets for the discovery of habitable exomoons or exomoons in general,” the authors write.
Astronomers haven’t had much success detecting exomoons, though there are several candidates. However, we may be on the verge of an initial confirmation. A research team of astronomers used the JWST to examine exomoon candidates but hasn’t published their results yet. The ESA’s upcoming PLATO mission may also be able to detect some exomoons.
Even though we only have simulation results for now, it seems impossible that our Solar System is the only one with moons. Exoplanets must also exist. Prior to the launch of Kepler, we were anticipating a wealth of discoveries. Now, we’re poised to learn much more about the exomoon population. Based on this research, we can expect some of these exomoons to be in habitable zones.
“We conclude that the circumstellar habitable zone can be extended to moons around giant planets,” the authors write.
Written by Evan Gough/Universe Today.
