16 planets found in Milky Way
2006-10-05 07:20
Paris - A seam of stars at the centre of the Milky Way has shown astronomers that an entirely new class of planets closely orbiting distant suns is waiting to be explored, according to a paper published on Thursday.
The finding opens up a new area of investigation for space scientists probing extrasolar planets - planets that orbit stars other than our own, it says.
Astronomers have spotted 202 extrasolar planets since the first was spotted in October 1995.
Their technique is based on the so-called "wobble" method. Under this, the astronomer measures the frequencies of light from the star. There is a telltale oscillation, or wobble, in this light if the star is tugged by a planet.
So far, planets with the shortest orbits, of 1.2-2.5 days, have mainly been found encircling stars that are hot and bright, with a mass at least three-quarters that of our Sun.
An international team of astronomers, using a camera aboard Nasa's Hubble telescope, delved into a zone of the Milky Way known as the "galactic bulge", thus called because it is rich in stars and in the gas and dust which go to make up stars and planets.
They uncovered the existence of 16 planets in the category of close orbiters, taking between 0.4 and 3.2 days to go around their respective stars.
Many of the planets are the size of Jupiter, the largest planet in our solar system.
Two of the 16 have orbits of less than a day, creating a new category of "ultra-short" orbits.
In addition, the planets generally orbit stars that are somewhat lighter than the typical stars seen in earlier extrasolar discoveries.
The team, led by Kailash Sahu of the Space Telescope Science Institute in Baltimore, Maryland, report their work in Nature, the British weekly science journal.
They believe that any planets which orbited at such a close distance to brighter, hotter stars would be destroyed by solar radiation.
And large Jupiter-sized planets are being spotted at these remarkably close orbits because the star, being of low mass, exerts a relatively low gravitational pull.