In Star Wars VI we first meet the Ewoks living on the Forest Moon of Endor. The planet Endor itself is a gas giant, but the Forest Moon is a habitable world, peopled by small furry sentient creatures. While we may not be living in the Star Wars universe, astronomers have now found the first evidence for a moon orbiting a gas giant planet in a star system other than our own.
The new study, published in Science Advances, reports signs of such an “exomoon” orbiting Kepler-1625b, which is a Jupiter-sized exoplanet. The planet goes around the yellow parent star every 287 days at the same distance as the Earth orbits the sun. The planet and moon are therefore likely to be in the “habitable zone” of the star, with equilibrium temperatures likely reaching 300-350 Kelvin (27-227°C).
However, before getting too excited about the prospect of finding Ewoks, it should be noted that the exomoon (formally named as Kepler-1625b-i) has a radius of around four times that of the Earth and a mass of around 16 times that of our planet, so is in fact similar in size and mass to the planet Neptune. It is therefore unlikely to have a solid surface and is most likely a largely gaseous body, like the planet which it orbits every 22 days.
The exoplanet was originally found by the transit technique, in which a planet passing in front of its host star, along our line of sight, causes the star’s brightness to dim slightly (by around 1% for a Jupiter-sized planet orbiting a sun-sized star) once every orbit. So such a dip in a star’s brightness is a “light signature” of an exoplanet.
Clearly, an exomoon may contribute an additional transit signal of its own, but for an object comparable in size to the Earth’s moon, the extra dimming will only amount to around ten parts per million – making it difficult to detect. This is further complicated by the fact that the moon signal will also be in a different location with respect to the host planet at each epoch, so the moon transits will not be strictly repeatable, and multiple moons around a single planet may wash out any observable signal anyway.
Jupiter itself has over 70 known moons, and four of these are comparable in size to our own, so multiple moon signals are not unexpected from exoplanet systems.
The first hints that there may be an exomoon in the Kepler-1625 system came in a paper by the same authors in 2017. In this study, they analysed 284 light curves from the Kepler satellite of planet-hosting stars that were considered to be plausible candidates for systems containing exomoons. They found no evidence for exomoons in the vast majority of the systems, but Kepler-1625 showed tantalising signs of the tiny dip in brightness that constitutes an exomoon signature.
In an attempt to confirm their original tentative detection, the team behind the new study scheduled high precision measurements of Kepler-1625 with the Hubble Space Telescope, which were carried out in October 2017. They observed the system throughout a predicted transit of the planet Kepler-1625b over the course of 40 hours.
After analysing their data, they found that the planet transit began 77.8 minutes earlier than predicted. This is interpreted as a so-called transit timing variation, caused by an unaccounted for gravitational tug on the planet by an unseen body. The same technique has previously been employed to determine the masses of planets in systems with multiple transiting planets, where each planet tugs the others, giving rise to such variations in timing. For example, the Trappist-1 system of seven transiting planets exhibits strong transit timing variations. These enabled astronomers to derive the masses of the planets directly from the transit light curve.
The transit timing variation seen in the Kepler-1625b data could similarly be due to the presence of an unseen outer planet, perturbing the orbit of the gas giant. Alternatively, it could be caused by an exomoon.
Compelling evidence for it to be an exomoon came when they noticed a slight transit – a dimming in brightness of around 0.05% – occurring just after the exoplanet transit. Moreover, this additional transit occurred at precisely the expected location to account for the size of the observed transit timing variation. As the authors point out, the combination of these two things “suggest that the exomoon is the best explanation”.
Is it really a moon?
Accepting that the data do indeed show evidence for an additional body co-orbiting the star with the planet Kepler-1425b, the question arises – is this really a moon? With a radius that’s around a third that of its parent planet, this object is unlike any moon of a giant planet in the solar system.
Even Saturn’s largest moon, Titan, has a radius that is less than 5% that of its parent planet, and while it’s true that the Earth and moon have a size ratio of about four to one, they are both rocky bodies. The Earth-moon system formed as a result of a giant impact in the early solar system. The moons of Jupiter and Saturn on the other hand coalesced from the debris left behind after the planets formed. Other moons, such as Neptune’s largest moon Triton, may have been captured from the Kuiper belt. Current theories cannot therefore explain how a Neptune-sized moon could have formed in orbit around a Jupiter-sized planet.
One of the authors behind the study – David Kipping from Columbia University in the US – has been speculating about the possible existence of exomoons, and describing how they might be detected, for the last ten years. He has led the field over this time, so I am delighted that his persistence has paid off. As with most “first” discoveries, this first exomoon detection is not yet absolutely conclusive, as the signals are at the limit of what is currently measurable, but I am hopeful that its existence will be confirmed with subsequent observations.
Moons are certainly plentiful in our solar system – the four giant planets host over 200 moons between them – so it is entirely plausible that many (or even most) of the currently known exoplanets – almost 4,000 – should harbour exomoons. And excitingly, some of the moons around the giant planets in our solar system – including Europa, Enceladus and Titan – are currently our best bet for finding life outside our own planet (or possibly on Mars).
There is already a lot of anticipation about the prospect of finding life on exoplanets. If we can also detect exomoons reliably, that possibility increases dramatically. It’s thrilling to realise that the next chapter of exoplanet discoveries has begun.