The first exolune, which scientists recently discovered in the constellation Cygnus, may be a giant super-Earth the size of Saturn. The history of her birth is still an absolute mystery.
“All the possible options for the properties of this moon are a huge mystery to theorists – this planet is too large for its formation to be explained by the classical theories of the birth of moons, and the data from Kepler and the history of the birth of the solar system do not give any clues in this attitude, “said Rene Heller from the Institute for the Study of the Solar System in Göttingen, Germany.
Well-known planetologists David Kipping and Alex Tychy reported on the discovery of the first satellite of the planet outside the solar system in July, when they shared with the public the results of a project to search for such objects in data collected by the Kepler telescope.
As scientists say, this moon revolves around the planet Kepler-1625b (a reduced analog of Saturn), whose radius is about half that of Jupiter, and six times greater than that of Earth. It makes one revolution around the star in about 287 days and is practically in the middle of the “zone of life”.
However, the latest measurements of the properties of the Kepler-1625 star show that it can be much larger than was thought. In this case, the Kepler-1625b will be similar not to Saturn, but to Jupiter, and its satellite will prove to be impossibly large-comparable in mass and diameter to Neptune, and not with Jupiter’s satellites.
Such discrepancies, as noted by Heller, prevent to determine the exact size and properties of the moon Kepler-1625b-I. So he tried to understand how it might look at different combinations of the size of the star and the planet around which it rotates, and find the most plausible option.
As shown by these calculations, no plausible option was found – at best, the moon Kepler-1625b will resemble the Earth or Venus, and at worst it will be a super-Earth the size of Saturn, completely covered by a multi-kilometer water ocean.
Most likely, Kepler-1625b-I will be something average, in size and chemical composition, reminiscent of Neptune or Uranus. And all three cases, according to Heller, can not be explained on the basis of modern theories describing the birth of the moon and the satellites of giant planets.
Planets of this size simply can not be formed either by the collision of the parent planet with a small celestial body, as our Moon emerged, nor as a result of the fusion of remnants of planetary “building materials”, as satellites of Saturn and Jupiter were born. Therefore, both the appearance of the first exoluna, and the possible history of its formation remain a mystery for scientists.
The first specific answers to these questions by Tichy and Kipping are expected to be received by the end of the week, when observations of the Kepler-1625b-I will begin with the help of Hubble. New images and data, as the scientists hope, will help them to clarify the size of the star and to understand whether their discovery is the first exotic “megaloon” or simply a large planet.