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Discovery of lightest known exoplanet April 21, 2009

Posted by CosmicThespian in Discoveries, News, Planet Gallery.
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A team of astronomers released two new bits of information on the Gliese 581 planetary system: one is  the discovery of a planet that is just two times more massive than Earth and the other is the realization that one of the worlds in this system is a candidate for hosting liquid water on its surface!

Gliese 581 is the name of a star 20.5 light years from the Sun in the constellation Libra.  That’s over 120 trillion miles away!  For the curious skygazer, Libra can be seen at this time of year rising in the east a little after 10 P.M; go take a look!

The star itself is what astronomers call a “red dwarf”.  The red color comes from the stars relatively cool temperature: about 5800 degrees Farenheit.  It’s also only about a third the mass of the Sun.  Red dwarfs are a favorite target of planet hunters because their lower mass means they are easier for planets to push around, thus giving them a larger wobble.  Think about what’s easier for you to push: a ping-pong ball or a basketball.  Apply the same force to both, and the ping-pong ball will go farther.  The same is true for stars and planets: put the same planet at the same distance from two different mass stars, and the lower mass star will respond with a larger wobble.  Larger wobble = easier to detect!

Gliese 581  has gained some noteriety in the past few years as the host of a multi-planet system; this most recent discovery brings the total number of worlds around this star to four.  The planets, known as b, c, d, and (now) e have masses 16, 5, 7, and 1.9 times the mass of the Earth, respectively.  The furthest planet out (d) orbits Gliese 581 in 66 days (Mercury, for comparison, orbits our Sun in 88 days).  The newest discovered planet is also the one closest to its sun; one year on Gliese 581 e is just over 3 days!  The astronomers responsible for this discovery believe that this world is most likely a rocky planet not unlike the four rocky worlds that orbit the Sun.  This remarkable discovery brings us one more step closer to the holy grail of exoplanet research: an Earth-like planet!

The same group was also able to refine the orbital parameters of another world in this system, Gliese 581 d.  In doing so, they found that this planet orbits in what astronomers refer to as the “habitable zone” of this star.  The habitable zone is the region around a star that is “just right” for liquid water to exist on the surface of a planet.  If a planet is closer than this region, it is too hot and water will simply evaporate existing only as steam; too far out, and water condenses into ice.  But at just the right range of distances, liquid water can flow.  As Stephen Udry, one of the astronomers on the team who announced these discoveries, says: “…it is the first serious ‘water world’ candidate.”

This diagram shows the distances of the planets in the Solar System (upper row) and in the Gliese 581 system (lower row), from their respective stars (left). The habitable zone is indicated as the blue area. (Image Credit: ESO)

This diagram shows the distances of the planets in the Solar System (upper row) and in the Gliese 581 system (lower row), from their respective stars (left). The habitable zone is indicated as the blue area. (Image Credit: ESO)

ESO Press Release.

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