Our sun sits in a rather lonely part of the Milky Way. Here in our quiet little home in a mundane part of the Orion Spur, a tiny offshoot from one of the galaxy’s spiral arms, we have few neighbours. Our night skies are dark and relatively empty. But perhaps the same may not be true of all planets in the galaxy, particularly after the recent discovery of planet bearing sun-like stars within the Beehive Cluster.
The Beehive Cluster is an open cluster of stars some 577 light years away from us here on Earth. Easily spotted with the naked eye, and quite beautiful when seen through a telescope, the Beehive is one of the nearest open clusters to us. Loosely bound by gravity, open clusters are groups of stars formed from the same molecular cloud, still too young to have drifted away from their siblings.
The two planets found in the Beehive, Pr0201b and Pr0211b, are hot jupiters (with orbital periods of 4.4 and 2.1 days respectively). While this means there aren’t going to be any dramatic scenes like the one depicted above, this is beside the point. The exciting fact here is that planets can and do exist in open clusters – a matter which had previously been under some debate. Because we, frankly, don’t know much about how planets form, the question of how planets may form in an environment as gravitationally disruptive as a star cluster is still a matter of much research and speculation. Until this discovery, there were some who were skeptical of whether planets could exist in such environments at all!
Well, evidently they can, which is good news for planet hunters. We know that stars often like to form in crowded environments, and now we know that planets can form in such environments too. Star clusters typically have a core of hot, bright stars, with a corona full of lower mass stars. Those lower mass stars include sun-like stars, and may apparently include a lot of planets. It has to be hoped that a discovery like this may bolster searches for planets in other open clusters too – there are plenty of open clusters in the Milky Way, including some well known clusters like the Pleiades and the Hyades.
The other thing this seems to imply is that planets are indeed more likely to form around stars which are richer in metals – a belief which I’ve long been an advocate of. After all, astronomically speaking, most of our planet is made of metals. Indeed, so are our bodies. So strong is the planet-metal connection that Russel White, the PI of this investigation, uses the lovely term of “planet fertilliser” when referring to such metals.
Now the question is when anyone might spot an Earth-like planet in a star cluster. Personally, I’d be willing to bet that terrestrial planets can exist in open clusters just like hot jupiters. For now though, let’s just wait and see!
Upper – all photography and manipulation by myself
Lower – NASA/JPL