Astronomers have found the second smallest exoplanet, HD156668b, a so-called “super-Earth” that’s just four times the mass of the Earth. With an orbit of just 4.6 days (compare that to Mercury’s 88-day orbit), this planet would not be a nice place for life. Yet, that such a small planet can be duly reported at AAS amid shoulder shrugging by the scientists and press alike shows just how far the exoplanet field has come in a few years.
The week of AAS began with discoveries from Kepler, and its search for transiting planets. And now it’s ending (or at least my blog posts are ending) with this discovery, which comes from astronomers using the twin 10-metre Keck telescopes in Hawaii to look for the wobble caused by the planet on its parent star.
The two bookends show not only the race between the space-based and ground-based observatories, but also the different approaches they use: transits, which give planet diameter, and the wobble searches, which give mass.
But as much as the two groups are competing, they also need each other. Not just to confirm planets — there are many false positives out there — but also to get both size and mass so that density can be calculated. Density, it turns out, is the crucial measurement that will tell us whether these distant worlds are puffed up and full of gas, buoyant and watery, or hard as rock. For now, all we know about HD156668b is that it is damn hot.
Image: L. Calcada