K2-25: eccentric hot Neptune with a mass of seven Earths

US, WASHINGTON (ORDO NEWS) — Of the approximately 4,300 exoplanets confirmed to date, about ten percent of them are classified as “hot Jupiters.” These are planets with masses from 0.4 to 12 Jupiter masses and orbital periods of less than 110 days (this means that they revolve around their star – usually much closer than Mercury to the Sun – and have a higher surface temperature). “Hot Neptune” has a smaller mass (about the same as our Neptune) – about twenty times lighter than Jupiter, and also rotates closely around its star.

Astronomers study not only the properties of exoplanets, but also how they developed in their planetary systems. Hot Jupiters and hot Neptunes are puzzles. It is expected that they formed much further in the cold limits of their systems, like the giant planets in our solar system, and then migrated inward to their current, nearby places. Evidence supporting this evolutionary story must be found in the orbital eccentricities of the planets and other clues, but it is difficult to obtain.

CfA astronomers Jonathan Irwin, David Charbonno, and Jennifer Winters were members of a team that investigated the evolution of the hot Neptune K2-25, a transit exoplanet with an orbital period of only 3.48 days, whose estimated mass is approximately equal to seven Earth masses, and a highly eccentric orbit (value 0 , 27; its maximum distance from the star exceeds the minimum distance by about 70%).

K2-25 has the advantage of being in a young star cluster, whose age is limited to approximately 650 million years. This young age checks whether there is time for the migration mechanism to work, whether it can make the planet leave with its large observable eccentricity, and also whether such a young star can be quite active (the star itself rotates in 1.88 days).

The team analyzed twenty-two inconsistent planet transits obtained using MEarth ground observatories, an IRAC / Spitzer mission camera and a Kepler mission, simulating each of the transits separately before combining the findings. According to their estimates, the period of time during which the orbit will become circular after migration is about 410 million years, which approximately corresponds to the age of the system, and, therefore, the fact that the orbit is eccentric suggests that some other body can outrage.

Scientists were looking for evidence of the presence of other planets in the system that could be responsible, looking for small changes in the transit light curves from K2-25 that could be the result of their gravitational presence (“transit time changes”). They did not find a single one.

The result, although it leaves room for ambiguity, is consistent with the theory that this hot neptune migrated into its system.

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