The smallest planet around a normal star other than the Sun may be even smaller than first thought. A new analysis suggests the rocky body weighs just 1.4 Earths - less than half the original estimate. Observations over the next few months should test the prediction.
Most known "exoplanets" are huge gas giants, hundreds of times Earth's mass, and were discovered by detecting the wobble they induce in their parent stars.
But in 2008, astronomers discovered a planet estimated to weigh just three Earth masses. Called MOA-2007-BLG-192-L b, it claimed the title of the lightest known exoplanet, apart from one tiny world discovered orbiting a dead star called a pulsar.
The planet was detected using a technique called microlensing, in which one star passes in front of another as seen from Earth. Light from the background star is gravitationally bent and magnified for a period of days to weeks during the event. But if the nearer star hosts a planet, the planet's gravity can give an added boost to the background star's light for a few hours.
Heavier star, lighter planet
Analysing these events takes time, because there are so many variables to take into account, including the sizes of planet and star, their separation, and the distance from Earth.
Initially, the team believed that this host star was a brown dwarf - an object too puny to sustain nuclear fusion, as normal stars do. That suggested MOA-2007-BLG-192-L b weighed 3.3 Earths.
But more recent observations suggest the parent star is actually heavier than first thought - a type of star called a red dwarf, team member Jean-Philippe Beaulieu of the Paris Astrophysical Institute reported last week at a meeting of the Royal Astronomical Society in London.
That suggests the planet weighs just 1.4 Earths. In size terms, that makes it a near twin of our own planet, closer in mass than any known planet except Venus.
'Biggest goals'
Scott Gaudi of Ohio State University in Columbus, who is not on the team, says the new measurements "give a much more robust estimate" of the mass of the planet and its host star.
"The result is important because this is the lowest-mass planet yet detected, and is extremely close to the mass of the Earth," he says. "Obviously, finding a true Earth-mass planet is one of the biggest goals of searches for exoplanets. We are very close to that goal now."
The team plans to get more data on the parent star in April or May using the Very Large Telescope in northern Chile.
If their analysis is confirmed, it is an unclear whether the tiny planet could host any life. Because its host is a very dim red dwarf, the planet is likely to be frozen - even though it orbits at about the same distance as Venus from our Sun.
However, if the planet boasts a thick, insulating hydrogen atmosphere, it could sustain a habitable surface temperature that might be able to support some life of some kind.
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Have your say
I Like Sciencetist To Be Very Good
Mon Jan 19 02:52:04 GMT 2009 by soklek HOUT
I need the new picture of sciece abdat
I Like Sciencetist To Be Very Good
Mon Jan 19 06:39:47 GMT 2009 by Nik
That's nice. Makes no sense whatsoever, but it's the thought that counts (or lackof it), right?
I like scientists to be capable, also. In fact, we all do.
What About Mercury?
Mon Jan 19 08:52:53 GMT 2009 by clueless
"Smallest known planet may actually be Earth-mass"
No... Mercury is smaller and lighter!
But you are referring to exoplanets.
What About Mercury?
Mon Jan 19 11:24:40 GMT 2009 by Dan Palmer
Hi Clueless,
You are wiser than your name seems to indicate...
Indeed the header and first line didn't make it clear that this was an exoplanet, so we have tweaked the words to reflect this.
Many thanks,
Dan Palmer, online sub-editor
What About Psr B1257+12 A?
Mon Jan 19 14:39:39 GMT 2009 by Antares
The innermost planet going around the pulsar named PSR B1257+12 is less massive than Earth.
According to Ken Croswell's book Planet Quest (page 153), this planet has about 0.019 Earth masses. This planet goes around its star every 25.34 days.
Two other planets, each with several times more mass than the Earth, go around this pulsar at greater distances. These two outer planets were discovered in 1991; the innermost one was found two years later, in 1993.
Page 152 of Croswell's book has an interesting diagram that shows the resemblance of these three planets' orbits to those of Mercury, Venus, and Earth. In particular, if you double the size of the pulsar planet system, each planet lines up almost perfectly with Mercury, Venus, and Earth.
I presume the New Scientist story means to say that the newfound world is the least massive extrasolar planet found orbiting a main-sequence star--as opposed to a pulsar.
Still, it's an interesting finding.
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