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The extrasolar planet MOA-2007-BLG-192Lb was discovered on 30 May 2008
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Correction:

"The New Scientist and perhaps other media outlets are reporting that the mass of MOA-2007-BLG-192Lb has been revised down to 1.5 Earth masses, but these reports are in error. The reporter has been confused by a report of one of my colleagues regarding a revision in the mass estimate that would be possible if the host star was confirmed to be a ~0.09 solar mass M-dwarf instead of a brown dwarf. The correct mass estimate remains 3.3 (+4.9 / - 1.8) Earth masses. This is currently the lowest mass estimate for an exoplanet except for PSR 1257+12 b, but the error bars have large overlap with a number of other planets detected by both radial velocities and microlensing" - David Bennett, for the MOA, OGLE, and PLANET collaborations.

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The smallest planet yet detected around an ordinary star 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.

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Researchers lead by David Bennett, University of Notre Dame, have discovered on of the smallest planets outside the solar system.
The researchers used the new MOA-II telescope at the Mt John Observatory, New Zealand, to measure the new planet was just three times bigger than the Earth.
The planet orbits around a "brown dwarf", that has 20th the mass of the Earth's sun, and is 3,000 light years from Earth.

"It turns out that the lowest mass ones are the ones that would be easiest to search for evidence of life on other planets" - David Bennett, University of Notre Dame, the leader of the international search team.

"No planets have previously been found to orbit stars with masses less than about 20 percent that of the Sun, but this finding indicates that even the smallest stars can host planets".

The astronomers used a technique called gravitational microlensing to find the planet, a method that can potentially find planets one-tenth the mass of our own.

"This discovery demonstrates the sensitivity of the microlensing method for finding low-mass planets, and we are hoping to discover the first Earth-mass planet in the near future"

The research will be published in the September issue of the Astrophysical Journal.

Adapted from Source

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MOA-2007-BLG-192Lb
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MOA-2007-BLG-192Lb is an extrasolar planet orbiting the brown dwarf or low-mass star MOA-2007-BLG-192L. At a mass of approximately 3.3 times that of Earth, it is one of the smallest known extrasolar planets.

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Position(2000): RA 18h 08m 04s, Dec 27 09 00

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Title: A Low-Mass Planet with a Possible Sub-Stellar-Mass Host in Microlensing Event MOA-2007-BLG-192
Authors: D.P. Bennett, I.A. Bond, A. Udalski, T. Sumi, F. Abe, A. Fukui, K. Furusawa, J.B. Hearnshaw, S. Holderness, Y. Itow, K. Kamiya, A.V. Korpela, P.M. Kilmartin, W. Lin, C.H. Ling, K. Masuda, Y. Matsubara, N. Miyake, Y. Muraki, M. Nagaya, T. Okumura, K. Ohnishi, Y.C. Perrott, N.J. Rattenbury, T. Sako, To. Saito, S. Sato, L. Skuljan, D.J. Sullivan, W.L. Sweatman, P.J. Tristram, P.C.M. Yock, M. Kubiak, M.K. Szymanski, G. Pietrzynski, I. Soszynski, O. Szewczyk, L. Wyrzykowski, K. Ulaczyk, V. Batista, J.P. Beaulieu, S. Brillant, A. Cassan, P. Fouque, P. Kervella, D. Kubas, J.B. Marquette

We report the detection of an extrasolar planet of mass ratio q ~ 2 x 10^(-4) in microlensing event MOA-2007-BLG-192. The best fit microlensing model shows both the microlensing parallax and finite source effects, and these can be combined to obtain the lens masses of M = 0.060 (+0.028 -0.021) M_sun for the primary and m = 3.3 (+4.9 -1.6) M_earth for the planet. However, the observational coverage of the planetary deviation is sparse and incomplete, and the radius of the source was estimated without the benefit of a source star colour measurement. As a result, the 2-sigma limits on the mass ratio and finite source measurements are weak. Nevertheless, the microlensing parallax signal clearly favours a sub-stellar mass planetary host, and the measurement of finite source effects in the light curve supports this conclusion. Adaptive optics images taken with the Very Large Telescope (VLT) NACO instrument are consistent with a lens star that is either a brown dwarf or a star at the bottom of the main sequence. Follow-up VLT and/or Hubble Space Telescope (HST) observations will either confirm that the primary is a brown dwarf or detect the low-mass lens star and enable a precise determination of its mass. In either case, the lens star, MOA-2007-BLG-192L, is the lowest mass primary known to have a companion with a planetary mass ratio, and the planet, MOA-2007-BLG-192Lb, is probably the lowest mass exoplanet found to date, aside from the lowest mass pulsar planet.

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An international team of astronomers led by David Bennett of the University of Notre Dame has discovered an extra-solar planet of about three Earth masses orbiting a star with a mass so low that its core may not be large enough to maintain nuclear reactions. The result was presented Monday (June 2) at the American Astronomical Society annual meeting in St. Louis.

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Astronomers have sighted the smallest extrasolar planet yet orbiting a normal star - a distant world just three times the size of our own.
Discovering a planet with a similar mass to that of Earth is considered the "holy grail" of research into planets that lie outside our Solar System.
It is vital because researchers want to find other worlds that could host life.
The new planet orbits a star which is itself of such low mass that it may be a "failed star" known as a brown dwarf.

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