Fantastic coincidence: a planet that shouldn’t be there at all

SAC scientists have discovered a new giant exoplanet orbiting a subgiant star so close that the planet ought to have been destroyed by tidal forces - but it isn't - at least not yet!

2016.06.07 | Ole J. Knudsen

The size of subgiant K2-39 and its exoplanet. K2-39b are shown relative to the size of the Sun. The distance between K2-39 and its planet is also indicated, relative to the distance of the Sun to Mercury. The Earth is not shown on this figure, because it is more than two times further away than Mercury.

Scientists working at the Stellar Astrophysics Centre have discovered a

new giant exoplanet, which orbits a subgiant star. Such stars have

evolved to become several times larger than the Sun. Around subgiant

stars, only a few short-period planets are known, and some scientists

have speculated this may be because planets get tidally destroyed as the

star evolves and grows larger. The new planet, named K2-39b, orbits

extremely close to its host star: it orbits at a distance of only 1.7

times the diameter of the star! By comparison, the Earth is more than

100 solar diameters away from the Sun. On the other hand, the subgiant

star hosting the planet is almost four times larger than the Sun.


The planet was discovered using the NASA K2 mission which detects

planetary transits, when planets move in front of their star relative to

a distant observed. Its mass was further measured using the Nordic

Optical Telescope in La Palma, the ESO 3.6m telescope in La Silla, and

the Magellan II telescope in Las Campanas Observatory. According to

Vincent Van Eylen, who led the study, the discovery is interesting

because it shows that such close-in planets can exist around evolved

subgiant stars. "Previously, we thought perhaps such planets would

quickly be destroyed due to tidal interactions once the host star

evolves. K2-39b has not been destroyed, or at least not yet, so either

the tidal destruction is not as efficient as we may have thought or

feared, or the planet may be destroyed in the next few thousand years.

But if that's the case, it would be an extreme coincidence that we found

it in the first place, and the effects of tidal decay should become

visible within the next few years."


Further studies of planets orbiting evolved stars will help understand

the fate of planets as their host stars grow older. The same will happen

to the solar system in a few billion years, when the Sun will evolve

into a giant star.


The discovery paper is accepted for publication in the Astrophysical

Journal, but the publication date is not yet known. A pre-print can be found HERE. For more information or


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