“There are currently 175 known moons orbiting the eight planets in our solar system. While most of these moons orbit Saturn and Jupiter, which are outside the Sun’s habitable zone, that may not be the case in other solar systems,” said Stephen Kane, an associate professor of planetary astrophysics and a member of the University of California Riverside’s Alternative Earths Astrobiology Center about the life-bearing potential of moons of planets beyond our solar system. “Including rocky exomoons in our search for life in space will greatly expand the places we can look.”
In a 2018 paper published in The Astrophysical Journal, researchers at the University of California, Riverside and the University of Southern Queensland identified more than 100 giant planets that potentially host moons that might be capable of supporting life. Their work will guide the design of future telescopes that can detect these potential moons and look for tell-tale signs of life, called biosignatures, in their atmospheres.
Since the 2009 launch of NASA’s Kepler telescope, scientists have identified thousands of planets outside our solar system, which are called exoplanets. A primary goal of the Kepler mission is to identify planets that are in the habitable zones of their stars, meaning it’s neither too hot nor too cold for liquid water—and potentially life—to exist.
Terrestrial (rocky) planets are prime targets in the quest to find life because some of them might be geologically and atmospherically similar to Earth. Another place to look is the many gas giants identified during the Kepler mission. While not a candidate for life themselves, Jupiter-like planets in the habitable zone may harbor rocky moons that could sustain life.
121 Giant Planets Identified
The researchers identified 121 giant planets that have orbits within the habitable zones of their stars. At more than eight times the radius of the Earth, these gaseous planets are less common than terrestrial planets, but each is expected to host several large moons, as Jupiter and Saturn do.
Scientists have speculated that exomoons might provide a favorable environment for life, perhaps even better than Earth. That’s because they receive energy not only from their star, but also from radiation reflected from their planet. Until now, no exomoons have been confirmed.
“Now that we have created a database of the known giant planets in the habitable zone of their star, observations of the best candidates for hosting potential exomoons will be made to help refine the expected exomoon properties. Our follow-up studies will help inform future telescope design so that we can detect these moons, study their properties, and look for signs of life,” said astronomer and lead author, Michelle Hill, currently at UC Riverside.
“It is very hard to find exomoons,” explains Rene Heller –a member of ESA’s upcoming PLATO mission who leads work packages to develop algorithms and statistical methods to find small, Earth-sized exoplanets, in an email to The Daily Galaxy. “One of the most promising ways is to look for their transits in front of their host stars. When an exomoon passes in front of its star, it blocks a tiny fraction of the star’s light, so that the star appears slightly dimmer than out-of-transit. “Slightly” here means a few or a few tens of parts per million.
“There haven’t been many detections of transiting giant planets in the habitable zones (HZs) around their stars,” Heller notes in his email. “In the exoplanet.eu online database I find about a dozen transiting Jupiter-sized planets in the HZs around sun-like stars. The planets (and their hypothetical moons) only transit their host stars once every few hundred days as seen from Earth. For an exomoon detection one would need to observe at least a handful of transits to be sure that the candidate signal is not caused by stellar or instrumental variability. So depending on how many transits there have already been observed for any of these transiting planets, it would take at least a few more years until we would have enough data to even find any moons around them.
Will be a Space-Based Discovery
“The key challenge, Heller concludes, is that one would probably need to observe any future transits from space (not from the ground) because one would need to continuously observe the star for at least, I would say, 20 hours. 50 hours would be better. That said, telescope time from space (e.g. with Hubble) is highly competitive and an exomoon survey is very risky – in the sense that it is thought to be very likely to return empty-handed. Hence, the needed observations are currently, in a way, unavailable.”
Astrophysicist Stephen Kane, wrote in an email to The Daily Galaxy: “ It’s a very challenging topic for several reasons. Firstly, Kepler and TESS use the transit method, which is heavily (!) biased toward short period planets. The Hill radius (radius of gravitational influence) of a planet scales linearly with orbital semi-major axis. For example, Jupiter has a Hill radius of 50 million kms and a semi-major axis of ~5 AU. Ganymede has a semi-major axis of 1 million kms. If we moved Jupiter to distances of ~0.5 AU or less, then it would likely lose most of its moons, including Ganymede. This means that many of the short-period planets and compact systems detected by Kepler & TESS do not have moons. I wrote that in a paper Worlds Without Moons.
“Secondly, we should not assume that habitable moons exist since that would require sufficient mass to maintain an atmosphere,” Kane notes. “There have been several studies that show the mass of moons are naturally truncated during the formation process and possibly stripped through dynamical processes during early migration:
“Having said all of that, we should definitely look to test all of these ideas,” Kane concludes in his email. “I think direct imaging will ultimately be the best approach, since relying on transits condemns you to see only the very rare cases where all the orbits are aligned, and the systems are probably far away in those cases. For solar system analogs, the aforementioned Jupiter-Ganymede always makes a great case study in terms of detectability since they’re the largest planet and moon we have locally to compare to. However, Titan is a great analog since it does actually have an atmosphere (~1.5 bars) and so interesting to study sustainability of atmospheres on moons.”
Avi Shporer, Research Scientist, with the MIT Kavli Institute for Astrophysics and Space Research via Rene Heller, Stephen Kane, and Exploring Kepler Giant Planets in the Habitable Zone
Image credit: An artist’s impression of an Earth-like moon in orbit around a Saturn-like exoplanet. NASA