Astronomers present renewed evidence of a moon in a distant star system. Orbiting a Jupiter-sized planet, it has planetary proportions according to the data: Kepler-1708 bi is believed to be 2.6 times larger than Earth. It is only the second hot candidate for an extrasolar planetary companion, because its detection remains difficult. If the sightings are confirmed, they will clarify that moons are typically among the planetary systems of the universe.
Our solar system is literally teeming with them: in addition to the planets, around 170 moons form the worlds of our cosmic homeland. Especially among the numerous satellites of Jupiter and Saturn, there are extremely exciting specimens: The moons Europa and Enceladus, for example, could have liquid water inside and are therefore even potential habitats. Our terrestrial moon is also assigned an important role in relation to life, at least indirectly: it stabilizes our world decisively. This directs the questioning gaze into the vastness of space. It can be assumed that many planets in distant star systems are also orbited by moons. However, solid evidence of this is still lacking.
On the trail of distant moons
“Astronomers have found thousands of exoplanets, but finding exomonds is a much bigger challenge. That’s why they’re still terra incognita,” says lead author David Kipping of Columbia University in New York. His team first presented an exomond candidate around the planet Kepler-1625b about four years ago. The astronomers found the evidence of the team by the so-called transit method. It is based on the fact that celestial bodies dim the light of their central star slightly when they pass it. The companion of a planet can of course only provide a particularly fine signal in the light curves. Nonetheless, the data provided evidence of a possible Neptune-sized satellite of Kepler-1625b. However, the existence of the exomond is still not confirmed beyond a doubt.
Kipping and his colleagues came across the second candidate for such a satellite as part of their further Exomond manhunt. They looked for traces in light curve data collected by NASA’s “planet hunter” Kepler space telescope. The focus of the scientists was on 70 gas planet candidates that orbit their respective star at a greater distance than between the sun and the earth. As the abundance of moons in Jupiter and Saturn in our solar system suggests, the best chances of finds can be expected from the outer major planets.
As the team reports, they encountered a suspicious signal in the case of the planet Kepler-1708 b. About the size of Jupiter, this gas giant orbits a sun-like star some 5,500 light-years from Earth in the constellations of Cygnus and Lyra. In the analyzed light curve data, the researchers found indications that it is not alone in passing its parent star. “It’s a stubborn signal,” says Kipping. It is best explained, according to the scientists’ analyses, by the existence of a large exomond orbiting Kepler-1708 b.
A huge companion is emerging
From the information so far, the suspected companion, designated Kepler-1708 bi, is about 2.6 times larger than Earth. This would make it about a third smaller than the previous Exomond candidate Kepler-1625 bi. According to the astronomers, both suspected moons probably consist of gas that has accumulated due to their strong gravitational pull. It also seems possible that they were initially planets that were then drawn into orbit around Kepler-1625b and 1708b and thus developed into their satellites. In addition to such giant moons, there are probably also many smaller moons – like in our solar system. “The first discoveries in a survey are usually the most conspicuous specimens – we find the large ones most easily with our limited sensitivity,” says Kipping.
As he and his colleagues themselves emphasize, further research is now needed to substantiate the latest results. For example, observations with the Hubble Space Telescope could provide more information. Because so far, questions remain unanswered. Eric Agol from the University of Washington, for example, comments on the latest indications: “It could just be a fluctuation in the data, which is either due to the star or to instrumental noise,” says the astronomer. But Kipping is optimistic and points out that the search for exoplanets initially delivered questionable results – similar to the search for exomonds today.
Source: Columbia University, professional article: Nature Astronomy, doi: s41550-021-01539-1