At just over 150 light years from Earth in the constellation Taurus, the Hyades is the nearest open star cluster to Earth. We’ve been scouting the terrain in clusters recently, looking at globular clusters like 47 Tucanae and open clusters like M37, both of which are under intense scrutiny. But the first exoplanet to be identified definitively in either kind of cluster seems to be Epsilon Tauri b in the Hyades [but see below re a 2.5 Jupiter mass planet in the system comprising pulsar PSR B1620−26 and its white dwarf companion — that one is in the globular cluster M4].
It’s an interesting world, a gas giant that’s a little less than 2 AU out with an orbital period of 1.63 years. This is the first planet discovered around a red giant, its star the most massive of all planet hosts known. [My mistake: several planets evidently orbit red giants — see comments below, and check here for another example]. That leads to intriguing speculation: Should we expect planets around other red giants, and do our current planet formation theories work in this environment?
The latter point bears watching. Greg Laughlin (UC-Santa Cruz) argues that in the case of Epsilon Tauri b, the cluster’s harsh ultraviolet radiation should have disrupted the protplanetary nebula.
The UV radiation environment in the original Hyades cluster was fierce. The protostellar disks of the individual Hyads were likely photoevaporated before the growing planetary cores were able to reach the runaway gas accretion phase that gives rise to Jupiter-mass planets (see our paper on this topic). When we get the full inventory of planets in the Hyades, I think we’ll find plenty of Neptunes and terrestrial planets, but almost nothing in the Jovian range. Indeed, work by Bill Cochran and the Texas RV group has demonstrated that the Hyades are generally deficient in massive planets.
So how do we explain Epsilon Tauri b? Laughlin thinks this may be an example of a planet forming via the process known as gravitational instability, which can produce massive planets and is little affected by nearby ultraviolet radiation. Gravitational instability is a model in which instabilities within the protostellar disk can cause gas giants to coalesce. The rival core accretion model sees such planets growing from small cores of rock and ice that acquire new mass through collisions, eventually growing large enough for their gravity to draw in nearby gas.
Laughlin is saying that the likely dispersal of the protostellar disks in the Hyades stars (thanks to UV) makes core accretion less likely in the case of Epsilon Tauri b (there simply wasn’t time). Whereas if gravitational instability produces a planet for every few hundred stars formed, as Laughlin believes, then there is no reason not to expect such a world in an open cluster like the Hyades. That’s a win for gravitational instability, though Laughlin still sees core accretion as the dominant model, writing elsewhere that “…the weight of observational and theoretical evidence seems to be shifting against the gravitational instability hypothesis.”
The paper is Sato et al., “A Planetary Companion to the Hyades Giant Epsilon Tauri,” accepted by The Astrophysical Journal but not yet available at the arXiv preprint site.