- A new kind of planet has been discovered in a star system
50 light years away. It is the smallest world to be found outside our Solar
System, and probably the first rocky planet found so far, astronomers revealed
on Wednesday.
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- The planet orbits the star mu Arae, which has been monitored
by a European team of astronomers using an instrument called HARPS (High
Accuracy Radial velocity Planet Searcher), attached to the 3.6-metre telescope
at La Silla observatory in Chile.
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- HARPS measures the frequency of light from the star,
so precisely that it can pick up slight doppler shifts caused by motion
of the star's surface. It is sensitive to velocity changes as small as
1 metre per second.
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- The team's original aim was to catalogue stellar pulsations
in order to work out the internal structure of the star, but they noticed
a regular oscillation that had to have another source - the star was being
wobbled by the gravity of a planet.
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- The new planet orbits about 13 million kilometres from
mu Arae, with a year lasting only 9.5 Earth days, and it has about 14 times
the mass of Earth. That gives it about the same mass as Uranus, the smallest
giant planet in our Solar System.
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- Ice or gas
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- However, team member Nuno Santos of Lisbon University,
Portugal, does not think the new planet is a gas giant. Being so close
to its star means it probably never had a chance to gather an envelope
of ice or gas. Instead, it is probably a rocky world, like Earth - just
a lot bigger. "It's some kind of super Earth," says Santos.
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- HARPS could spot smaller worlds too. "I think we
can go below 10 Earth masses; it's only a question of time," Santos
told New Scientist. But detecting true twins of the Earth will probably
require a completely different method, because a star's own pulsations
would obscure the gravitational wobble from such small planets.
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- The newly discovered planet must be extremely hot. Its
star, mu Arae, is similar to the Sun, but the planet orbits at less than
a tenth of Earth's distance, so it gets a hundred times as much heat and
light.
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- http://www.newscientist.com/news/news.jsp?id=ns99996329
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