The Hungarian Automated Telescope Network (HATNet) is run out of the Harvard-Smithsonian Center for Astrophysics, with primary stations at Mauna Kea (HI) and on Mt. Hopkins (Arizona). Looking at large fields of stars over many consecutive nights, the automated telescopes involved help astronomers identify the periodic dimming that marks a transiting exoplanet. And with other projects like the Trans-Atlantic Exoplanet Survey (TrES), the XO Project in Maui and the Optical Gravitational Lensing Experiment (OGLE) in Chile operational, transit data are piling up.
All of which is useful indeed, for at present we are just nearing 20 transiting planets detected from ground-based observatories. The discovery paper for the most recent HATnet detection makes an interesting point about the size of our sample, declaring it “…still small enough that individual discoveries often advance our understanding of these objects signiﬁcantly by pushing the limits of parameter space, either in planet mass, radius, or some other property.”
How true, and a corrective to the notion that we can make quick generalizations about the planets thus far found. At a recent cocktail party, I was button-holed by a man who wanted to know whether or not most solar systems are going to be like our own. I had to explain that the data aren’t remotely available to make such a call, that we are beginning to know some things about particular classes of planets, especially those most readily detected by radial-velocity methods (and early in the data gathering process at that), but that due to the nature of our techniques there are planetary regimes we know little about, including outer gas giants and terrestrial planets in habitable zones.
Transits are a particularly useful kind of detection because astronomers can get good information about the mass and size of such planets, as well as having a shot at learning something about their atmospheres. The new HAT find is designated HAT-P-3b, a planet that turns out to be the smallest transiting world found photometrically (the transits of smaller planets have been noted — among them, GJ 436b — but not before their original detection by radial-velocity methods). The star in question is the K dwarf GSC 03466-00819, now called in the paper HAT-P-3.
Its planet, HAT-P-3b, orbits at a scorching 0.03894 AU, and looks to be about 60 percent Jupiter’s mass, with a radius almost 90 percent that of Jupiter. Theory suggests that radius is consistent with a core of heavy metals of about 75 Earth masses, a finding supported by the metal content of the host star. The paper is Torres et al., “HAT P-3b: A Heavy-Element Rich Planet Transiting a K Dwarf Star,” scheduled to appear in Astrophysical Journal Letters, with preprint available.