British astronomers claim to have detected light coming from a planet outside our own Solar System. If the claim holds up, it will represent the first window on worlds beyond our Solar System.
Since the first discovery of a planet outside our Solar System four years ago at least 30 'extrasolar' planets have been reported, six of which were added to the list only last month. Most were detected 'indirectly', their presence inferred from the wobbles they induce in the stars they orbit.
A planet exerts a small gravitational tug on its star, despite being much less massive than it, which causes the star to wobble as it orbits the centre of mass of the star-planet system. This tiny motion produces a periodic Doppler shift in the frequency of the light coming from the star, similar to the pitch change of the noise of a passing car. So very careful measurements of a star's light can betray the presence of its planets.
But to induce a measurable shift, a planet has to be close to its parent star and very large. This is why all the extrasolar planets seen so far are Jupiter-like giants. Such worlds may not be especially common in the galaxy, they are simply the only ones we can spot. Sadly, they are probably all balls of gas-not promising as locations where life might have evolved.
The case for the existence of these planets is now overwhelming; but it would still be very exciting to see one of them directly, rather than seeing only its effect on its sun. Starlight reflected from an extrasolar planet would carry vital clues about the planet's composition, just as the oxygen-rich atmosphere of Earth can be deduced from spacecraft measurements of the sunlight it reflects.
Now Andrew Collier Cameron of the University of St Andrews, Scotland, UK, and colleagues, report what could be the first direct detection of an extrasolar planet. As they explain in
Cameron's team studied the planet orbiting the nearby star, τBootis. The planet, discovered in 1997, was originally thought to be at least four times the mass of Jupiter. It traverses its orbit once every three days; in other words, it experiences a three-day 'year'. So this is a world unlike any in our Solar System-it is a giant much closer to its star than small, rocky Mercury is to the Sun. At that proximity, it will be ablaze with reflected light, making it a good candidate for direct detection.
But the planet-scattered starlight is outshone 10,000–20,000-fold by the brilliance of τBootis itself. So filtering the planet's light from the star's is immensely difficult. The authors believe they have achieved this feat by exploiting the very different Doppler shift in the two signals, whose sources are moving in different ways.
They deduce that the planet is about eight times the mass of Jupiter, and nearly twice the size-larger than predicted by theories of giant-planet formation. The planet also appears to be bluish green.
In a similar study recently, David Charbonneau and colleagues at the Harvard-Smithsonian Center for Astrophysics, Massachusetts, were unable to find any sign of reflected planetary light in the starlight of τBootis. Thus, other astronomers are likely to be cautious about the new results unless they can be independently confirmed.
Interestingly, several groups, including Charbonneau's, have very recently reported the direct detection of another extrasolar planet, from the very slight dimming of the light from its star as the planet passes in front.