You might think I’m going to discuss the newly discovered explanet around Proxima Centauri, but – no – that’s pretty much been done to death. Instead, I thought I might tell you about our paper that was accepted on the same day.Our paper is about the Kepler-20 system. In 2011, it was announced that the Kepler spacecraft had found 4 planet candidates in what is now known as the Kepler-20 system. The following year, 3 were validated (Kepler 20 b, c and d) and it was announced that there may also be two Earth-sized planets in the system. These two were confirmed (Kepler-20e and f) later the same year.
The Kepler satellite uses the transit method for detecting exoplanets. This essentially looks for small periodic dips in a star’s brightness to infer the presence of an orbiting object. From this, you can determine the object’s radius, but not its mass. For that, you need to use the radial velocity, or Doppler wobble, method, which I discussed in this post. Some of the early studies did do some radial velocity measurements and did present mass estimates for a few of the planets, but they were not accurate enough to infer much about their composition.
What we did was observe the system with HARPS-N. The observant among you may already have noticed that the figure above has 6 planets, not 5. Yes, we discovered one more. A ~20 Earth mass planet, located between Kepler-20f and d. It wasn’t found earlier because it doesn’t transit, so we don’t know the radius. However, given its mass, it is probably similar to Neptune. You may also have noticed, that the figure above also compares the orbits to that of Mercury; all 6 planets orbit closer to their parent star than Mercury is to the Sun.We also managed to get a more accurate estimate for the mass of Kepler-20b. It’s a ~1.9 Earth radius planet with a mass of ~9.7 Earth masses. This gives it a density of 8.2 g cm-3. If you look at the mass-radius relation figure on the left (Kelper-20b is the orange data point labelled K-20b), it lies on the same composition curve as the Earth and Venus. It is, therefore, the most massive rocky planet known.
Recently, it’s been suggested that most planets with radii of 1.6 Earth radii, or greater, are not rocky. Here we have one that it 1.9 Earth radii, and apparently rocky. It is, however, orbiting very close to its parent star and, therefore, heavily irradiated. It’s possible, therefore, that this is a planet that once had a significant gaseous envelope that it has lost due to photo-evaporation; we are maybe seeing the bare core of what once was a gaseous planet. It is therefore still possible that rocky planets do not typically form with radii above 1.6 Earth radii.
Anyway, that’s some of my exoplanet news for the week. Maybe not quite as exciting as the new planet around Proxima Centauri, but still interesting, even though none of the planets are potentially habitable (the host star is similar to the Sun) and even though it is almost certainly too far away to consider sending anything to visit the system 🙂 .