A Western University team might have spotted six exomoons lurking in old astronomical data.
The find, if confirmed and published in a peer-reviewed journal (it is currently awaiting publication), would be an exciting one. There are something like 4,000 exoplanets – or planets outside our solar system – found since the 1990s. But exomoons remain an elusive target, and none have been confirmed yet. And that’s not for a lack of trying.
Exomoons are small, making them hard to spot in telescopic data. Usually planets are spotted by looking at the amount of light they block from a star, or how much of a tug on the star they produce by gravity during planetary orbits. Exomoons, being so much smaller, hardly show up in this data. Some tentative finds have been discussed over the years, but nothing found for sure.
But the Western team says they might have the key in looking at old NASA Kepler space telescope information. Kepler was optimized to look at planets orbiting distant stars, performing its mission between 2009 and 2018 (when it ran out of fuel). So far astronomers have found something like 2,300 planets in the Kepler data – and more may come with further study.
The key in spotting the potential exomoons came from looking at a metric known as “transit timing variations” – or how much change there is between when a planet is supposed to pass across a star, and when it actually does. In theory, the subtle gravitational tug of a moon may slightly disturb the orbit of its parent planet. That gravitational disturbance could alter the planet’s orbit in its path around the parent star, but only by a few minutes.
The Western-led team first went back to publications of transit timings from the Kepler archive from two papers: a 2016 Astrophysical Journal Supplement paper led by Tel Aviv University’s Tomer Holczer, and a 2017 Monthly Notices of the Royal Astronomical Society paper led by Columbia University’s Jingjing Chen and the well-regarded planet-hunter David Kipping, also from Columbia. Using these established archives let the Western team automatically remove events masquerading as planetary transits, such as stellar flares or sunspots.
From this archive of thousands of planets, the research team narrowed the group down to 100 or so candidates with small transit variations for further investigation, before selecting six with potential exomoons.
“We’re kind of right at the very edge of Kepler’s limits in these cases,” said lead author Chris Fox, a Western Ph.D. candidate working with professor Paul Wiegert. “We’re measuring these signals on the order of 10 minutes or less, but sometimes we have some significant error in that, of five or so minutes. We’re kind of in the weeds a little bit, but that was the whole point of the project.”
The results ferreted out six potential exomoons that are very large, compared to what we are used to in our solar system. The smallest moon of the group is estimated at 10 times more massive than Earth’s moon. The largest moon would be five times the size of our planet. As one can imagine, the planets these potential moons are orbiting are accordingly quite large, in the mini-Neptune to super-Neptune range.
Could the moons be habitable? It seems unlikely. Even though most of their masses are large enough to potentially hold onto an atmosphere or exosphere, Fox said, each of these systems are extremely close to their parent stars.
The shortest planetary orbital period is 42 Earth days and the longest on the order of 150 days, putting these planets – and their possible moons – well inside the habitable zone of their stars where liquid water would exist on their surfaces. Instead, the water might very well evaporate.
At worst, the atmospheres might also be at threat if the stars are very active and blast radiation into space. Fortunately, none of the observed stars are the very active M-dwarfs, which are prone to these periodic storms. Most of the stars are sun-like in mass, which we assume would be fairly stable environments. One star trends a little smaller in the K-dwarf range, and a couple of other stars are larger than the sun in the subgiant range.
The system that appears most comparable to Earth is Kepler 409b (KOI-1925.01). It probably wouldn’t be a habitable system, with an orbit of 69 days; Mercury’s, by comparison, is about 88 days. Kepler 409b appears to be about the mass of Earth, but the proposed moon is huge. The mass of that moon is 20 percent the size of the Earth-sized planet. Earth’s moon, by comparison, is only one percent the mass of our planet.
“This system is very similar to Pluto-Charon, but scaled up proportionally,” Fox explained. The dwarf planet Pluto has several moons; its largest, Charon, is about 20 percent the mass of Pluto. But Pluto is a pipsqueak in extraterrestrial worlds, at only 0.2 percent the mass of our own planet.
Is confirmation of these exomoons possible? Fox says it depends on the capabilities of telescopes that currently are meant to look for distant planets – not moons. The forthcoming James Webb Space Telescope (which may launch in 2021 at best, depending on how the pandemic affects its schedule) is a sort of all-purpose telescope that can look at many things, including planets. Whether Webb can confirm these exomoons is thus an unknown, Fox said.
There are other possibilities for confirmation, Fox added. NASA’s Transiting Exoplanet Survey Satellite may be able to look at the closer-in systems, which are hundreds of light-years away. The European Space Agency’s forthcoming CHaracterising ExOPlanet Satellite (CHEOPS) might also be able to investigate, if the luminosity of the potential exomoons isn’t too limiting for its photometry.
Alternatively, a telescope optimized to look at radial velocity data from gravitational tugs may provide an independent confirmation, although Fox said he is not too familiar with which ones might be helpful. (A powerful example of radial velocity instrumentation is HARPS, the High Accuracy Radial velocity Planet Searcher at the ESO La Silla 3.6-metre telescope in Chile, but Fox said his expertise is more in transit-searching technology.)
If the exomoons are confirmed, Fox said the discoveries will help us better understand planetary formation more generally. “We only have one data point [for moons] – our own solar system,” he explained. “We don’t know if our solar system is common because it has so many giant planets with moons, or if that is unusual.”
Fox also pointed to an analogy with early planet-hunting. If these newly discovered potential exomoons seem startlingly large, it’s helpful to remember that the first exoplanets found in the 1990s tended to be Jupiter-sized or larger planets that were extremely close to their parent stars, where signals were easier to find. Gradually, planet-hunting technology has evolved to find Earth-sized planets (or even smaller planets) further from their stars. A similar revolution could be expected with exomoons as telescope technology continues to improve.
The six exomoon candidates are in the star systems known as KOI 268.01, Kepler 517b (KOI-303.01), Kepler 1000b (KOI-1888.01), Kepler 409b (KOI-1925.01), Kepler 1326b (KOI-2728.01) and Kepler 1442b (KOI-3220.01).
The findings were recently submitted to the Monthly Notices of the Royal Astronomical Society.