IN THE 19th century astronomers spent a lot of time seeking shadows crossing the sun. They were searching for Vulcan, a putative planet inside the orbit of Mercury, by looking for its transits. These are the moments when, viewed from Earth, the hypothetical planet would cross the solar disc. Sadly, there was no Vulcan to be found, but the method itself is sound, and it is the modus operandi of Kepler, an American spacecraft that has been trailing the Earth, in the same orbit, since March 2009.
Kepler is a telescope that looks simultaneously and continuously at more than 150,000 stars, recording the amount of light coming from them. It is seeking the tiny, periodic diminutions of illumination caused by planetary transits and, on January 4th, the team running it announced that five such patterns had shown up in the first six weeks of the probe’s operation.
The past 15 years have shown that planets are commonplace. More than 400 have been located around stars other than the sun, by looking for the wobbles in parent stars that orbiting planets cause. A decent wobble, though, requires a massive planet, so the wobble method does not favour the discovery of Earth-sized objects. Kepler, however, can find such planets. The Earth itself, in transit, reduces the amount of light an observer would see from the sun by about 0.01%. That is well within Kepler’s range.
In fact, the planets found so far are significantly larger than Earth. Four are about the size of Jupiter and one about the size of Neptune. They also have much shorter orbits, ranging from 3.3 to 4.9 terrestrial days. Neither of these facts is surprising. Even using the transit method, big planets are easier to spot than small ones, and to be sure that a flicker in brightness is caused by a planet rather than some property of the star itself, it must occur at regular and predictable intervals. Hundreds of flickers that might have been caused by planets with longer orbits have been seen, but have not yet have been confirmed as transits.
What this does mean, though, is that the planets in question are are much closer to their stars than Earth is, and thus much hotter (1200-1650ºC), as well as being larger. But they are not as hot as the most peculiar discoveries Kepler has made. These are two planet-sized objects that are far hotter (at 12,000ºC) than their distances from their parent stars suggest they should be. That means they are giving out energy of their own, yet they are too small to be stars. One theory is that they are youngsters, giving off heat as they collapse inwards due to the pull of their own gravity, but nobody knows for sure.
None of these discoveries favours the underlying reason why planet-hunting is such a popular sport—the hope that, one day, a life-bearing planet will turn up. For that, more numbers will have to be crunched, and planetary atmospheres analysed for signs of oxygen. The hunt, however, is on in earnest. If Earth-sized planets are out there, they will soon be found.