Professional and amateur astronomers working together have found a young exoplanet in the Hyades open cluster. The planet is weirdly large, given its host star.

red dwarf in Hyades
Astronomers have detected a transiting exoplanet around the red dwarf star K2-25 (circled) in the Hyades open star cluster. This view from the Digitized Sky Survey shows Gamma Tauri (brightest star), which sits near the base of the "horns" of Taurus. The Hyades cluster is the closest open cluster to Earth.
Andrew Mann / McDonald Obs. / DSS

The family unit of stars is the open cluster. These loose stellar families can contain a few hundred to a few thousand members, all born at roughly the same time. Well-known examples are the Pleiades (about 110 million years old) and the Hyades (650 to 800 million years old).

Open clusters behave a lot like human families: at first they’re close, but with time, members spread out and lose touch. But even when they do, they still have the same traits when it comes to age and chemical composition.

Astronomers have found a handful of exoplanets in open clusters, primarily through the radial velocity wiggle the planet induces in its host star’s position as it orbits. Open cluster planets are interesting because they’re young, and also because many stars, including (probably) the Sun, were born in these associations. So studying these environments gives us a look at the “normal” childhoods of stars and planets.

The repurposed Kepler spacecraft, now under the mission moniker K2, looked at the nearby Hyades cluster as part of its new lease on life. K2 data are freely available for any curious researcher to explore. Amateurs Daryll LaCourse and Thomas Jacobs dug around, and they found what looked like an exoplanet passing in front of a star in the Hyades data. They reported it to professional Andrew Mann (University of Texas at Austin), whose team followed up with various surveys and a new infrared spectrograph called IGRINS on the 2.7-meter Harlan J. Smith Telescope at McDonald Observatory.

Sure enough, the transit signal looks like a planet. The star is a young, spotty red M dwarf, but the spots don’t explain the repeat transit event. The researchers couldn’t find any stellar companions that could be the culprit, either. The team estimates in the February 10th Astrophysical Journal that the planet, K2-25b, is about 3½ times larger than Earth (about the size of Neptune) and orbits its star every 3.48 days. But they couldn’t determine the mass: the star is so “jittery” at this age that IGRINS can’t detect the tiny radial velocity wiggle.

This exoplanet is not the first found in the Hyades, but it’s the first detected in the cluster by its star-transiting signal. The star, K2-25, is about 16th magnitude and could be visible with an 18- to 20-inch scope under good skies (the coordinates are right ascension 04:13:05.61 and declination +15:14:52.00 if you want to try).

What’s interesting about K2-25b is how big it is: it’s 10% the width of its parent star. No other planet has been found around an M dwarf that’s so large compared with its star. The team suspects the planet might be puffed up and losing its primordial atmosphere due to the radiation pouring out of the young, active star. Astronomers will need two pieces of information to check this theory: one, the planet’s mass — and the instruments that could detect it are now coming online, Mann says; two, whether planets in other young systems are also abnormally large. K2 has also observed the Pleiades and Praesepe (a.k.a. Beehive) open clusters, so hopefully more worlds will turn up in the data.


Learn more about the discovery from McDonald Observatory’s press release, or the team’s Astrophysical Journal paper.


Reference: A. W. Mann et al. “Zodiacal Exoplanets in Time (ZEIT). I. A Neptune-sized Planet Orbiting an M4.5 Dwarf in the Hyades Star Cluster.” Astrophysical Journal. February 10, 2016. Full text here.

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