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Smallest planet recorded

Media Release
3 June 2008

Smallest planet recorded

Astronomers have discovered a tiny star with its own planet, 3000 light years away.

The star, the planet’s equivalent of the Sun, is the smallest star on record to have an orbiting planet. It has a mass about 6% of our sun and is so small it may be incapable of producing energy by nuclear reactions. The planet is slightly larger than Earth, has a smaller orbital radius, similar to Venus, and, due to the small size of the star, is likely to be colder than Pluto.

The discovery was made by the Japan-New Zealand Microlensing Observations in Astrophysics (MOA) collaboration. A paper describing the result has been accepted for publication in the Astrophysical Journal, and the result is being presented today at the American Astronomical Society Annual Meeting in St Louis.

The star is a brown dwarf, and is likely to be up to one million times fainter than the Sun, leaving the planet’s atmosphere colder than Pluto. However, its location favours the presence of a massive atmosphere underlain by a deep ocean on its surface. It is possible that interior heating by radioactive decay would be sufficient to maintain the ocean at liquid temperature, yielding a possible habitat for life.

“The discovery indicates that that even the lowest mass stars can host planets” says Dr David Bennett of Notre Dame University in the US, the sole member of the MOA collaboration not from New Zealand or Japan. “Planets have not previously been found orbiting stars with masses less than about 20% of that of the Sun, but this finding suggests that we can expect to find other very low-mass stars to have planets with a mass similar to that of the Earth.”

The discovery was made possible by the new Japanese-funded MOA telescope at Mt John Observatory in Canterbury. This is the world’s largest telescope dedicated to gravitational microlensing which takes advantage of the fact that, as Einstein predicted, a star warps the space surrounding it, enabling the star to act like a giant magnifying glass. The telescope is equipped with a state-of-the-art CCD imaging camera capable of imaging an area of sky 13 times the size of the full Moon in a single exposure. With this powerful setup, the MOA collaboration is able to find rare instances where the magnification caused by gravitational microlensing is very high, and the sensitivity to planets enhanced.

Dr Ian Bond of Massey University leads the sophisticated computer analysis needed at the Mt John Observatory. He notes “the new MOA telescope-camera system is a superb instrument that can monitor an area of the sky containing more than 50 million stars in searching for these rare planetary microlensing events. We could not have made this discovery without it”.

The vast amount of data generated was analysed at several contributing institutions, including The University of Auckland. Undergraduate student Yvette Perrott carried out important computations for the discovery that confirmed the low masses of both the host star and the orbiting planet.

The MOA group is made up of astronomers from Nagoya University, Konan University, Nagano National College of Technology, and Tokyo Metropolitan College of Aeronautics in Japan, as well as Massey University, The University of Auckland, Mt John Observatory, the University of Canterbury, Victoria University in New Zealand, as well as Dr David Bennett of Notre Dame University. Additional astronomers included the Warsaw University Observatory in Poland, the Universidad de Concepción in Chile, the University of Cambridge, UK, the Institut d’Astrophysique de Paris, the Observatoire Midi-Pyr´en´ees, the Observatoire de Paris in France, the European Southern Observatory in Chile, and Heidelberg University in Germany.


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