The Hubble telescope managed to spot a large moon near a distant exoplanet thousands of light years away.
There are eight large planets and about 200 of their satellites in the solar system. Outside it, there are about 4,000 planets, and no satellites: it is too difficult to find them at such large distances. Passing along with the planet against the background of their star, they make a very weak contribution to the change in its brightness, moreover, they follow a more complex trajectory and give a less clear signal. However, if the Solar System is not something exceptional in the Galaxy, then the number of exoons should be an order of magnitude greater than the number of exoplanets - and sooner or later we will learn to observe them.
The first such example is provided by the exoplanet Kepler-1625 b: back in 2017, data appeared that made it possible to suspect the presence of a satellite. These observations have been confirmed in new work, the results of which are published in the journal Science Advances. Alex Teachey and David Kipping of Columbia University report the discovery of an exosatellite by the Hubble Space Telescope.
On October 28 and 29, 2017, scientists sent it to Kepler 1625 and observed it for 40 hours, including the 19-hour transit of the planet against the background of a star. Following this period, another stage of a weak decrease in the star's brightness was discovered. To trace it to the end, astronomers did not have enough observational time, but this did not stop to associate this effect with the movement of an exosatellite flying behind the planet. In addition, the transit of the planet Kepler 1625 itself began noticeably earlier than the estimated time, which apparently indicates the influence of the gravity of a neighboring and rather large body on it.
The transit of an exoplanet with a satellite against the background of a star changed its brightness, which was noticed by the sensitive Hubble telescope.
All these data Teachy and Kipping quite confidently associate with the presence of an exoluna, called Kepler 1625b i, or "Neptmoon": the satellite's dimensions are tens of times larger than our entire planet and are comparable to the size of Neptune. The planet itself is several times heavier than Jupiter, and the mass of its moon is 1.5% of the mass of the planet. These dimensions, by the way, make many experts still doubt the correct interpretation of the observational data.
In fact, if you focus on the solar system, there are three possible scenarios for the appearance of a satellite: from matter ejected into orbit as a result of a collision of a planet with a celestial body, from an asteroid captured by gravity, or from matter left over after the formation of the planet. So far, it is difficult to imagine that any of these mechanisms could work in the case of a satellite of such an impressive size.
Sergey Vasiliev
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