Four Stars and One Planet

Artist depiction of PH1. Credit: Ron Miller

A really cool new planet discovery has been announced today and I wanted to mention it here for those who haven't heard about it yet.
You may recall the Kepler Space Telescope has been starring at a patch of sky to look for periodic dips in the light of distant stars. Such dips can be caused when planets orbiting those stars transit in front of the star (just like our own Venus transit a few months back). There are so many stars to look through, though, that there is a public program available for anyone interested to look at the data. That's right: ordinary people can look at scientific data and help find planets. This Planet Hunters program is ongoing and has already discovered several planets.
Today, a really cool planet has been discovered by these citizen scientists. Read on to learn more.

PH1 is a planet 6 times the radius of the Earth and no more than half Jupiter's mass (probably much less). This places it at something like a Neptune, though much hotter. It orbits the system every 137 days, or a distance of 0.6 astronomical units (AU; for comparison, Venus's orbit is ~0.7 AU). However, instead of orbiting one star like most known planets do, this one orbits two. The two stars are an F-type star a bit larger than the Sun and a smaller M-dwarf somewhat smaller than the Sun. These two stars orbit each other every 20 days at a distance of 0.2 AU. The planet joins the growing list of circumbinary planets discovered with Kepler.

Orbital configuration of the PH1 system. The planet is labeled as 'b' in blue. Credit: Schwamb et al. 2012.

Another interesting aspect of the planet is the way in which it was discovered. Kepler has been starring at ~100,000 stars and it takes a while to look through each one. While we can sometimes write computer programs to search for the signatures of a transiting planet, nothing beats the human eye in terms of pattern recognition. Enter the Planet Hunters program. This is a citizen science program developed to look at Kepler light curves and allow ordinary people to see if they can find anything interesting in them. Two citizen scientists, Kian Jek and Robert Gagliano, first identified PH1 and notified the professional astronomers through the Planet Hunters network. It was soon confirmed that they had found a new circumbinary planet. If you have some time to spare, perhaps you should check out that program (and others like it) and try to make your own discoveries.

The Planet Hunters interface showing the transit of the planet as well as the primary and secondary eclipses. The primary eclipse occurs when the small, faint star blocks out the bright primary; the secondary eclipse is when that faint star goes behind the primary. Credit: Schwamb et al. 2012.

A circumbinary planet discovered by amateur astronomers is cool, but what makes the system yet more interesting is the fact that it has yet another pair of stars orbiting the first! It is a quaternary star system: 4 stars bound together. The system is divided in two binaries: the main one, Aa-Ab, with the planet orbiting around it, and the second one, Ba-Bb, ~1000 AU away. The second binary's primary (Ba) would be as bright as the full moon as seen from PH1; the fainter one (Bb) would require binoculars to spot amidst the bright glare of Ba.
This is the first planet found in a 4-star system, and while it's too hot to support life (given the proximity to the Aa-Ab binary), there may be other, smaller, potentially cooler planets in the system.

This is not the first time quaternary systems have been seen. HD98800, depicted below, is a famous 4-star system in the ~8 million year old TW Hydrae Association. One of the two binaries is known to host a disk of dusty material around it, perhaps left over from the planet formation process. While no planets are known (yet?) to exist in this system, it suggests that nature can still produce the right environment for planets to form in systems with multiple stars.

HD98800, a 4-star system with a circumbinary dusty disk around one pair. Could this be how PH1 formed? Credit: NASA/JPL-Caltech/T. Pyle (SSC)

We already knew binary stars, and stars in multiple systems in general, are quite common. Discoveries like this are showing that planets, both around single and multiple stars, are also very common. It may only be a manner of time before we find a 6-star planetary system like that described in Isaac Asimov's Nightfall.