Kepler’s done it again; this time the space telescope has found 6 exoplanets orbiting a distant star called Kepler 11, but this mini-solar system is vastly different from ours, and may have implications for the search for alien life.
This is the largest number of planets Kepler has found orbiting one star, and may be the most found by any telescope (the HD10180 system, found by the La Silla Observatory in Chile, may have seven stars, but we’re waiting for confirmation). Other than this, what makes this find so special?
1. The planets are orbiting very close to their star and close to each other; five of the planets orbit closer to their star than Mercury does to our Sun, and the sixth would obit between Mercury and Venus. Astronomers have already found many planets orbiting this close (see Hot Jupiters), but never before have they found so many planets packed into such a tight region. Astronomers have always expected to find most of a solar system’s mass further out, around the orbit of Jupiter and beyond, so this was a very surprising discovery and may change theories of solar system formation and evolution.
2. The planets have unusual densities; although the five inner planets are all larger than the Earth, between 2.3 and 13.5 times larger, they all have lower densities, suggesting that they aren’t made out of rock and metal like the inner planets of our Solar System. The outer planet is likely similar to Neptune, with an icy-core surrounded by hydrogen and helium gas, whilst the inner two planets have densities suggesting they are primarily made of water, with a thin atmosphere of hydrogen and helium gas, these may be examples of the hypothesised ocean worlds. The middle two planets have densities lower than the inner one so may also be composed of a mixture of ice and hydrogen-helium gas. As these planets are orbiting so close to their sun, they will be extremely hot (Mercury has a surface temperature as high as 430°C) and so astronomers would not have expected to find planets with such compositions surviving so close to their stars, another big surprise. The planets most likely formed past the ice-line and migrated inwards. Maybe they were larger ice or gas giants and lost most of their mass to evaporation as they approached their Sun (that last bit is totally my own speculation).
3. The density of the planets was been calculated with a new technique. Kepler finds exoplanets by looking for temporary dips in a star’s brightness caused by planets crossing in front of the star (see this post for details), a bit like your cat running in front of the TV causes a temporary drop in its brightness (and ruins that crucial scene in Quatermass). The larger the exoplanet (and cat), the greater the drop in brightness, and so astronomers can use this to work out the width and therefore the volume of an exoplanet. Usually another method is then used, called Doppler spectroscopy (see this post), to work out the mass of the exoplanet, and these two measures can then be combined to estimate the exoplanet’s density (mass / volume = density). But this solar system was too far away (roughly 2000 light years) and the exoplanets too small to use Doppler spectroscopy. Instead the team looked at the orbital dynamics of the planets; because they’re packed so close together the planets cause gravitational disturbances as they pass each other that cause their orbits to change slightly. The size of these disturbances is related to the mass of the planets, thus with some probably mind-bending maths the team could estimate each exoplanet’s mass and therefore their density. This is a great new method to detect exoplanets, particularly useful for those at great distances, although it would only work in limited situations, where multiple planets are orbiting closely to each other.
So what about the aliens?
Although the planets are orbiting a star similar to our Sun, none of them are likely candidates for life, at least life as we know it, as the planets will be too hot. The holy grail of finding the first exoplanet capable of supporting Earth-like life is still out there, but can’t be too far off (fingers crossed!)
The paper doesn’t discuss implications for alien life, but that’s my area of interest, so I wont let that stop me. Two of the key probabilities in the famous Drake Equation are the chance that other stars in our Galaxy will have planets, and the chance that some of these planets will be habitable. The fact that Kepler appears to be finding an abundance of exoplanets is great news, this may mean that planets are relatively common throughout our Galaxy and thus we may expect this to increase the likelihood that life will exist elsewhere as more planets equals more potential habitats. However, this discovery may also suggest that our Solar System is not the only type of solar system possible, in fact there may be many types, and ours may not even be the standard model. If this is the case then this may decrease the chances of alien life existing, as it will reduce the overall number of habitable planets in our Galaxy, especially if a high proportion of solar systems are like Kepler 11’s. Our Galaxy may be full of planets, but many of these may be in quite different solar systems from ours, and thus incapable of supporting life. Good news and bad news then.
This is all still in the realms of speculation until we know more, but this discovery does raise the possibility that there may be a number of bizarre solar system types out there in the Milky Way. Either way, the Universe keeps surprising us, which is best.
To be ‘referency’, here’s the link to the paper in Nature