Using NASAs Kepler spacecraft on its K2 mission, astronomers have discovered five new planets around other stars, including two extraordinary companions to a subgiant star.

A Neptune-mass exoplanet. Image credit: NASA / Goddard / Francis Reddy.

In the January 2017 issue of the journal Monthly Notices of the Royal Astronomical Society (arXiv.org preprint), Dr. Alexis Smith from the German Aerospace Center Institutes of Planetary Research and co-authors report the discovery of a giant planet and a brown dwarf orbiting K2-99, a very iron-rich star with a mass of 1.6 solar masses.

K2-99 is on its way to becoming a red giant. Our Sun will reach this phase in five billion years, once it has fused all of its hydrogen into helium, the researchers said.

Also known as EPIC 212803289, K2-99 is an 11th magnitude subgiant star in the constellation of Virgo, approximately 1,970 light-years away.

This star is being orbited by a Jupiter-like planet, K2-99b. But in contrast to Jupiter, which needs 12 years to complete one orbit around the Sun, K2-99b orbits its star in just 18 days, Dr. Smith said.

Until now, only a few transit planets have been found orbiting such subgiants.

The interesting thing about K2-99 is that we also see signals from a second object in a long-period orbit of several hundred days (K2-99c) perhaps a brown dwarf, he added.

Brown dwarfs are of great interest because they fill the evolutionary gap between planets and stars and are considered to be failed stars, which we still do not know much about.

In a paper available on arXiv.org, Dr. Philipp Eigmller, also from the German Aerospace Center Institutes of Planetary Research, and co-authors report the discovery of a hot gaseous planet around the star K2-60, also known as EPIC 206038483.

Dubbed K2-60b, the planet has a radius of 0.68 Jupiter radii and a mass of only 0.43 that of Jupiter.

This planet orbits its star in a mere three days, Dr. Eigmller said.

For comparison, it takes the innermost and fastest planet in the Solar System, Mercury, 88 days to complete one orbit around the Sun.

In the same paper, Dr. Eigmller and colleagues report the discovery of a giant planet orbiting the F9-type star K2-107, also known as EPIC 216468514.

The planet, named K2-107b, has a mass of 0.84 that of Jupiter. Its radius is 1.44 times that of Jupiter.

In a paper published in the Astronomical Journal (arXiv.org version), the same team reports the discovery of a warm Neptune-like planet in a 10-day orbit around the F-type star K2-98 (EPIC 211391664).

In contrast to the ice giant Neptune, however, this planet named K2-98b must be very warm due to the proximity to its star, Dr. Eigmller and co-authors said.

Because of this proximity, K2-98b will be engulfed by its own star in approximately three billion years, when K2-98 has become a red giant.

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A.M.S. Smith et al. 2017. K2-99: a subgiant hosting a transiting warm Jupiter in an eccentric orbit and a long-period companion. Mon Not R Astron Soc 464 (3): 2708-2716; doi: 10.1093/mnras/stw2487

Philipp Eigmller et al. 2016. K2-60b and EPIC 216468514b. A sub-Jovian and a Jovian planet from the K2 mission. arXiv: 1611.03704

Oscar Barragn et al. 2016. K2-98 b: A 32-M(Earth) Neptune-sized planet in a 10-day orbit transiting an F8 star. AJ 152, 193; doi: 10.3847/0004-6256/152/6/193

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