A Jewel for the Northern Crown

On approximately May 18 each year, the constellation Corona Borealis, a.k.a. the Northern Crown, is directly overhead as viewed from Berkeley, California (slightly North of zenith as viewed from my parents' place in Pasadena). I have to admit that I never paid much attention to this fairly inconspicuous constellation until about a year ago. But around the Summer 2006 I noticed that one of the stars in the constellation, designated kappa by the German astronomer Johann Bayer in the early 1600's, did something really interesting. From about 2004 to 2005 the star was steadily accelerating away from the Earth. This is not unusual, stars commonly do this as they are tugged by the gravity of other stars. But in 2005 kappa Coronae Borealis turned around and started heading back toward us. If it performed another of these turns anytime within the next 10 years, it would indicate that the star has a planet orbiting it.

I try not to get my hopes up too much when it comes to potential planets. I used to jump out of my seat every time I saw a hint of a signal in my data, but I've had far more false alarms and disappointments than planets over the years. Astronomy is characterized by long stretches of monotony interrupted by short stretches of amazing discovery. Most days I'm tracking down bugs in my computer program, or testing new algorithms, or writing telescope proposals. But as I noted in a previous post, every once in a while you get to figure out something really cool that no one else has ever known before. And if you're a planet hunter, sometimes you find new planets--new places in the Galaxy that no one on Earth ever knew about.

This summer I had one of those exciting Astronomy moments. Soon after I graduated from UC Berkeley, kappa Coronae Borealis performed a U-turn and revealed the presence of a Jupiter-like planet. The standard convention is to name planets after the stars they orbit. The first planet gets a "b" tacked onto the star's name, the second discovered gets a "c" and so on. This leads to a lot unromantic planet names. Instead of Tatooine or the Dagoba System, examples of extrasolar planets include HD86081b and HIP14810c. But because the kappa star in Corona Borealis is bright enough to be seen by an astronomer in the early 1600s, it gets named after its constellation--and so does this new planet. So I'm proud to announce today the latest exoplanet: kappa Coronae Borealis b.

My collaborators and I wrote a paper announcing the planet back in September (along with another planet, HD167042b) and it was accepted in the Astrophysical Journal about a week ago. I posted the the paper on the astronomy preprint server (astro-ph) and it appears in tonight's addition. Since the paper is fairly technical, here are the key points about this new planet:

1. It has an orbital period of 3.3 years, so a year on this planet would take 1208 days.
2. It orbits at a distance of 2.7 times the Earth-Sun distance, which makes its orbit remarkably similar to the minor planet Ceres in our Solar System, which orbits between Mars and Jupiter.
   
3. The planet is 1.8 times the mass of Jupiter, or 572 times the mass of Earth. It likely does not have a solid surface, but instead is surrounded by a deep atmosphere of hydrogen making it a rather unpleasant place to hang out. However, there's always the possibility that it has one or more moons that could be Earth-like. But until technology makes several more leaps forward, we can't know for sure.
4. The star has a mass of 1.8 times the mass of the Sun. Unlike the Sun, the star has run out of hydrogen fuel in its core and is now burning Helium. As a result, it has swollen to a radius of 4.7 times the radius of the Sun, it's about 12.3 times as bright as the Sun and 800 degrees cooler. If you were orbiting kappa CrBb, the star would appear to be about twice the size of the Sun as viewed from Earth (1 degree across, rather than 1/2 of a degree), and it would have a red tint, rather than the Sun's whitish-yellow. Evolved stars like kappa Coronae Borealis are known as subgiants.
5. Planets around these massive subgiants appear to be twice as abundant as planets around Sun-like stars. 9% of stars that are twice as massive as the Sun have Jupiter-like planets, compared to about 4-5% of Sun-like stars. This is evidence of a relationship between the mass of a star and the likelihood that it has a planet, which may be an important clue about how planets form.
6. The planet's orbit is not circular. Instead it has an eccentricity of 0.145. Here's how the orbit of the planet would look if it were in our Solar System:

This Spring, if you're ever in a dark place far away from a major city you should try to locate the constellation Corona Borealis. Right above the U-shaped crown is a red Jewel that has an alien world orbiting it. Who knows, maybe there will be some Kappans are looking back at you!

Now excuse me, but I have a proposal to edit and a deconvolution algorithm to test.