Second moon-candidate outside our Solar System

Exomoons are moons that orbit planets in other planetary systems. The  first exomoon candidate, Kepler-1625 b-i, was discovered in 2018 by Dr. Teachey & Prof. Kipping (Columbia University). The second, new moon candidate, Kepler-1708 b-i is very large, 2.6 times the radius of Earth. Its host planet is Jupiter-sized, and the moon orbits only 12 planetary radii from it. The planet- oon system is 1.6 times farther from their star than Earth is from the Sun.

Moon formation theory tells us, that most of the moons can only be found in the outer planetary systems, beyond a few-times the Earth-Sun distance. This is also what we see in our own Solar System: the terrestrial planets have only 3 moons (Phobos & Deimos of Mars, the Moon of Earth), while almost 200 moons can be found around the gaseous planets in the outer Solar System. In most cases moons form in disks around nascent gaseous planets, which type of planets can only form in the cold, outer planetary systems. This is the reason why most of the moons are expected to be very far from the central star. The first two exomoon candidate, however, are in the inner planetary system, giving headaches for the moon formation theorists about their origin. Among many possibilities, it could be that Kepler-1708 b-i and Kepler-1625 b-i formed as planets around the star, and got captured by their host planets, as they migrated closer to their star. Or, it is also possible that they formed in the disk around the nascent gas giant planet, and migrated together with their host planets into the inner planetary system. We need more data about these systems to narrow down the possibilities for the origin of these new moon-candidates.

Read more about the new discovery in:

• texternal page he scientific article
  
• in an outreach article in English at external page Scientific American
• in an outreach article in German at external page Tages Anzeiger