Life on a Star Trek planet?
Almost every week there is a news item about an ‘exoplanet’ – a planet outside our Solar System. Astronomers have now found over 500 of these planets orbiting around distant stars. The first exoplanets to be found were very odd compared to our familiar Solar System planets: they are big like Jupiter, but so close to their star they are baked to temperatures in excess of 1000°C. Naturally, most of us are more interested in whether there are planets like our own Earth out there.
One of the best ways to find out about exoplanets is to watch as they ‘transit’ across their host star. In our own Solar System, we can see the transits of Venus and Mercury as they pass between us and the Sun. During a transit of Venus, a Venus-sized portion of the Sun’s disc is obscured. If an exoplanet’s orbit happens to be lined up so the planet passes exactly between us and the host star, a similar event occurs. In the case of Venus and Mercury we can actually see the shape of the Sun and the transiting planet, but for exoplanets all we would see on a photograph, even with the most powerful telescopes, is an unresolved point of light. This is because even the nearest star to the Sun is very, very distant. But we can still detect an exoplanet transit.
The Kepler satellite is a space telescope designed to precisely measure the brightness of hundreds of thousands stars over and over again. By doing this, it reveals the small drop in brightness which occurs when a small exoplanet blocks a small portion of the light from the host star. In this way, it reveals transiting exoplanets. Kepler has been performing its repetitive measurements for almost two years now: if there were a suitably-oriented Earth-twin orbiting one of the huge numbers of Sun-like stars being measured, Kepler should have seen two transits. For closer-in planets, the orbit is smaller and a circuit around the star is completed more rapidly. Because of this, it takes less time to find the very hot, close-in planets, which is one of the reasons they were found first.
The most exciting announcement from Kepler so far is Kepler-10b, a small rocky planet which orbits a star similar to the Sun every 20 hours. This discovery is the first definitely rocky planet we have found outside our own Solar System, the first exoplanet a spaceship could land on. Kepler-10b would certainly not be a good place for Captain Kirk to take a stroll though: it’s far too hot and probably has no atmosphere at all.
Artist concept of Kepler-10b
Finding cooler transiting planets takes years. Once Kepler has three years of repetitive measurements, the Kepler scientists will be able to confidently identify exoplanets at similar distances from their host stars as Earth is from the Sun. These planets are likely to have similar temperatures to the Earth, and could possibly support life as we know it. On 1 February 2011, Kepler will make its second public data release. This will triple the amount of data which are publicly available with three months of measurements for 165,000 stars. Transiting exoplanets with orbital periods of a month or less will be revealed.
NASA, and the scientists involved in Kepler, are keeping the longer streams of fresher data secret for now, but it is possible that an Earth-like exoplanet might be revealed by their analysis quite soon! After the four-year mission, the Kepler results will tell us whether Earth-sized planets in Earth-like orbits are common. These statistics will tell us whether it is likely we live in a ‘Star Trek‘ universe, where planets like our own are plentiful.
Kepler will not quite have the final word in this adventure though. The stars Kepler is studying are distant from us, so they appear dim. This will make detailed studies of the exo-Earths Kepler reveals rather limited. The European Space Agency (ESA) has a mission called PLATO under consideration which will find the nearby exo-Earths: by studying a large fraction of the brightest stars in the sky, the PLATO mission should find many of the nearest Earth-twins. These planets will be much easier to study in detail, and will be the first destinations for the real-life Captain Kirks.
Study at the OU