Last night I was thinking that the day would come when all the planets we’ve been discovering have proper names instead of stark designations in catalogs. Then I realized that this is unlikely. As the rate of planetary discoveries accelerates through space-borne missions and ever more precise detection methods here on Earth, it may be that we’ll keep generating new finds faster than the naming process can catch up with them. So I guess we should get used to designations like Gliese 581 c.
Of course, a planet can have multiple designations, depending on how it’s catalogued or found. The recently announced gas giant HAT-P-2b (a very strange place indeed) is called this not for its place in a catalog but its discovery method, the HAT network of automated telescopes. HAT stands for Hungarian-made Automated Telescope, but the project is headquartered at the Harvard-Smithsonian Center for Astrophysics (CfA) and works with instruments in Arizona, Hawaii and in this case, Israel. Its focus: The detection of transiting exoplanets like this one.
But HAT-P-2b is also HD 147506 b, referring to its star’s listing in the Henry Draper Catalog. You can see that designations like these lack a certainty poetry, depending on the catalog or project involved (think of TrES-1, designating the Trans-Atlantic Exoplanet Survey that found it — examples begin to multiply). We’ll have to draw our aesthetic satisfaction not from colorful names but the exotic places these designations conjure up. And HAT-P-2b by any name turns out to be exotic indeed. Gaspar Bakos (CfA) calls it “…a really weird planet.”
The parent F-class star, HD 147506, lies 440 light years away in the constellation Hercules. Larger and brighter than the Sun, it’s home to at least one planet and perhaps another. That thinking is based on the odd elliptical orbit of HAT-P-2b, which represents a marked break from the circular orbits thus far found with transiting gas giants, all of which have been ‘hot Jupiters.’ With an orbital period of a scant 5.63 days, HAT-P-2b comes into transit every five days and 15 hours. Its oval orbit moves it from periastron at 3.1 million miles to fully 9.6 million miles at apastron, an eccentricity that is probably the result of an outer planet, although at this juncture there is no hard evidence of that planet’s existence.
This one is no hot Jupiter; in fact, it’s much closer to a failed star. The co-author on the discovery paper, Dimitar Sasselov, says this: “With 50 percent more mass, it could have begun nuclear fusion for a short time.” Even so, HAT-P-2b is one dense world, eight times the density of Jupiter, but evidently packed into a ball only slightly larger than that planet. It stands out compared to previously identified transiting exoplanets.
From the discovery paper:
Its mass of 8.17 ± 0.72MJ is ~5 times greater than any of these 14 other exoplanets. Its mean density ρ = 6.6 ± 2.7 g cm−3 is ~5 times that of the densest known exoplanet (OGLE-TR-113b, ρ = 1.35 g cm−3 ) and indeed greater than that of Earth (ρ = 5.5 g cm−3) or other solar system rocky planets. Its surface gravity of 149 ± 13 m s−2 is 5 times that of any of the previously known TEPs, and 20 times that of HAT-P-1b.
A mean density greater than Earth’s, and notice that last sentence, which points out the surface gravity is five times that of any transiting exoplanets (TEPs) previously found. Expect no media buzz with this one. A massive gas giant in a peculiar orbit around a hot F-class star doesn’t offer much by way of habitability potential. But most planets aren’t going to be habitable, and every transit we find offers the chance to measure the planet’s physical size from the degree to which the star’s light is dimmed by its passage. The paper is Bakos et al., “HAT-P-2b: A Super-Massive Planet in an Eccentric Orbit Transiting a Bright Star,” submitted to the Astrophysical Journal, with abstract available.