Usually planetary satellites are much smaller than our planet, but this time it's a body so big that it looks like a solid candidate for habitability.
An international team of astronomers has found a new celestial body in data from the Kepler Space Telescope. It is a satellite of a giant planet, but it is unusually large and apparently belongs either to the class of so-called super-Earths, or even warm Neptunes. Super-Earths, like warm Neptunes, are usually called planets that are noticeably larger than our own, but in this case we are not talking about a planet, but a satellite. Once again, it seems, the astronomical classification is at risk of becoming a little more complicated. A paper has been published in Nature Astronomy.
The new body has been named Kepler-1708 b-i. The first part of the name means it is in the Kepler-1708 b planet system, where "b" is the first planet discovered in the Kepler-1708 star system. It is a sun-like star around which the giant, Jupiter-like planet Kepler-1708 b orbits at a distance 1.6 times the distance of Earth from the Sun.
The planet, like most known exoplanets, was found by transit. That is, astronomers have recorded variations in the luminosity of the star Kepler-1708 and have found that it periodically falls to the same magnitude. However, a careful study of the luminosity graph has shown that there are still some fluctuations in the trough in it, corresponding to the planet obscuring the alien star. After careful analysis, the authors of the new work concluded that these must be explained by the presence of this planet's moon, and an unusually large one at that.
To cast doubts once and for all, scientists have tried to simulate such types of astrophysical activity of a star in the studied system, in which it could give similar fluctuations in the luminosity diagram and lead the observer to the false assumption that the planet of this system has a satellite. They carried out a total of 200 simulations and only two of them found signals which could indeed be confused with an exoluna similar to the one they discovered. So, the probability of the discovery being wrong is in the region of 1%.
In other words, one of the most reliable candidates for large exolunas has been discovered this time. And Kepler-1708 b-i is indeed a large satellite: its diameter is about 2.61 times that of Earth. At the same time, it orbits at a distance of about a million kilometres from the giant host planet, which means the latter must occupy a huge part of the visible sky on this planet - much larger than the Moon on Earth.
Previously, astronomers had found candidates for exolunas larger than the Earth, but rather few, and the probability of error there was noticeably higher than 1%. That is why many people regarded that data with disbelief. The new discovery shows that giant exolunas appear to be real. Although the mechanisms of formation of such large planetary satellites are still unclear. As is why they are rare or why they are not in the solar system. Here, remember, the largest satellites of giant planets are multiples of Earth's size.
The question of the habitability of an open giant exolune is unclear at present. In principle, a fairly massive body 240 million kilometres from a star like the Sun could be habitable if it has a dense enough atmosphere. At 2.61 times the size of Earth, Kepler-1708 b-i may indeed have a dense atmosphere, and possibly a magnetic field. A combination of both could give the planet not only an acceptable temperature for life, but also protection from the radiation rings of its huge host planet. In the solar system, a number of satellites of Jupiter and Saturn, lying close to their radiation belts, are unsuitable for human landings as they have murderously high levels of radiation danger.
