MONDAY January 6th passed uneventfully on planet OGLE-TR-56b. As usual, it was beastly hot, at 1,600°C, and it was raining molten droplets of iron. Dusk brought no relief, because there was no dusk. The planet's sun-like star never sets—and it is so close that it looms as large as a dinner plate held at arm's length.
Meanwhile, on a cooler planet located one spiral arm farther out from the centre of the galaxy, in the unsunny city of Seattle, astronomers were announcing OGLE-TR-56b's discovery. The planet has the fastest and smallest orbit of any of the 100 or so Jupiter-like gas balls that have recently been found orbiting stars other than the sun. Its year—the time taken to complete one orbit—lasts only 29 hours. Its parent star probably pulls hard enough to synchronise OGLE-TR-56b's spin with its orbit (much as the earth does to the moon), causing one side of the planet to be in perpetual daylight.
Ogling the sky
Of greater significance is how the new planet was discovered, according to Dimitar Sasselov of the Harvard-Smithsonian Centre for Astrophysics, in Cambridge, Massachusetts, who made the announcement at the American Astronomical Society meeting in Seattle. Its presence was betrayed by a decrease in light from OGLE-TR-56b's parent star when the planet moved in front of that star. This is a technique that could eventually reveal hundreds more planets, including small, rocky ones similar to the earth.
Such tiny specks of rock can never be found using the traditional method of planet-hunting, the “Doppler technique”. The signal that astronomers habitually sought is the to-and-fro wobble of a star caused by the tiny gravitational tug of an orbiting planet. A mass as puny as the earth's would not move a star any more than the random boiling motions of a stellar surface. However, an earth-sized obstacle would reduce the star's light by 0.01%. This may sound small but it is large enough to measure.
On that scale, spotting OGLE-TR-56b was a cinch; a giant by terrestrial standards, it covers 1% of the face of its parent star. This dip was first noticed by Andrzej Udalski and his group at Warsaw University. They included it in a list of 59 possible cases of stars with planetary eclipses (known as “transits”) that the group found while monitoring the brightness of 50,000 stars using a telescope in Chile. Dr Udalski's group, called OGLE (the Optical Gravitational Lensing Experiment, a name that reflects the original, and different, purpose of the survey), made their list public in February 2002.
A race then began to work out which of the 59 cases were genuine planetary transits, and which were false positives: eclipses due to companion stars, for example. The winners of the race, Dr Sasselov, together with Guillermo Torres and Saurabh Jha, his colleagues at the Harvard-Smithsonian, and Maciej Konacki of the California Institute of Technology, used the Doppler technique to work out the mass of the obstructing object in each case. In one system they confirmed that the object was unambiguously a planet.
Although Doppler confirmation was required, the transit method may turn out to have wider application. The Doppler method requires intensive monitoring by the world's largest telescopes, and works only with the brightest stars. Transits can be spotted using telescopes no bigger than those available in department stores. This means that the transit method can be used on fainter and more distant stars, increasing the volume of space that can be searched for planets by a factor of more than 10,000. An example of things to come, OGLE-TR-56b is the most distant planet known, almost 5,000 light-years away, as well as being the hottest and closest to its parent star.
That increase in range makes up for the fact that only planets with orbital planes edge-on to the earth's can be detected by the transit method. And although false-positive transits still have to be ruled out using the Doppler method, those follow-up measurements need not be as precise as the measurements needed to detect something with the Doppler method in the first place. As a result, they take much less time—a crucial advantage, because big-telescope time is, to an astronomer, more precious than gold.
Too close for comfort
What of OGLE-TR-56b itself? Many of the Jupiter-like planets found previously by the Doppler method also hug their stars closely. At first these discoveries were surprising. It was thought a gas ball such as Jupiter could not condense in such hot surroundings. What is more, the pattern does not conform to the assumed normal pattern of rocky inner planets and gaseous outer planets (as found in the earth's solar system). Now, astronomers have come to terms with the idea that it is our local arrangement that is unusual. Although the gas balls certainly form some distance from a star, where things are reasonably cool, they then spiral inwards due to interactions with the discs of dust and gas that surround stars for a few million years after they are born.
This idea could explain most of the properties of the newly-discovered planets. Among these properties was the finding that many planets have orbits as small as 4% of the diameter of the earth's but none has an orbit smaller than that. This, the theory went, was because the new star's intense radiation drove away any dust and gas within such a range, preventing planets from spiralling any further inwards. Unfortunately for the theorists, the orbit of OGLE-TR-56b is 2.5% of the diameter of the earth's—so they must now go back to the drawing-board.
New gas balls are always welcome. The transit method's real prize, though, would be the detection of earth-sized bodies. Despite the method's sensitivity, this will require the use of space-based telescopes, to iron out the effects of the earth's atmosphere. Both NASA, America's space agency, and ESA, its European counterpart, plan to launch telescopes dedicated to this task by 2007. If earth-like planets are common, these telescopes will find dozens of them, as well as hundreds more Jupiter-like gas balls.
The logic of transit searches might also help those searching for radio signals from extraterrestrial civilisations. Adam Kampff, an undergraduate at Harvard, says that the aliens most likely to transmit signals towards the earth would be those who know that the earth exists. And they will be the aliens who are in the right places in the galaxy to detect the earth transiting the sun. Aim your radio dish at stars which view the earth's orbit edge-on and you have a better chance of making contact with anything that is out there.