Sub-Mercury-sized exo-planet, another first for Kepler

21 February 2013

Sydney Institute for Astronomy researcher and Physics Head of School Professor Tim Bedding and Dr Dennis Stello have contributed to an international team in discovering the first sub Mercury-sized exoplanet, the innermost of three planets that orbit the Sun-like host star, Kepler-37. The results will be published in the prestigious journal Nature on February 28, 2013.

An artist's impression of Kepler 37 compared to the three smallest planets in our own solar system.
An artist's impression of Kepler 37 compared to the three smallest planets in our own solar system.

Owing to its extremely small size, similar to that of Earth's Moon, and highly irradiated surface, Kepler-37b is very likely a rocky planet with no atmosphere or water, similar to Mercury.

The detection of such a small planet shows for the first time that stellar systems host planets much smaller as well as much larger than anything seen in our own Solar System.

The Kepler spacecraft was launched in 2009 with the goal of determining the frequency of rocky planets in the habitable zone around Sun-like host stars in our Galaxy. Over 150,000 stars are continuously monitored for transits of planetary bodies. Over the course of 978 days of observations by the Kepler spacecraft, transit signals of three planets were detected in the photometric time series data of a star designate Kepler-37, a slightly cooler and older star than our sun.

"While sub-Mercury-sized planets are expected from theory, detection of tiny planet Kepler-37b is remarkable given a transit of this depth and period is potentially detectable for less than 0.5% of stars observed by Kepler." said Professor Bedding.

"That we now have discovered one of these small and hard-to-detect planets suggests that they are abundant around other stars, it does lend weight to the belief that planet occurrence increases exponentially with decreasing planet size" said Dr Stello.

The finding supports theories that occurrence increases exponentially with decreasing planet size.

Since the discovery of the first exo-planet we have known that other planetary systems can look quite unlike our own. However, until recently we have only been able to probe the upper range of the planet size distribution. The high precision of the Kepler space telescope has allowed us to detect planets that are the size of Earth and somewhat smaller, but no planets have been found that are smaller than those we see in our own Solar System.

Asteroseismic analysis of Kepler 37, conducted by the team including Professor Bedding and Dr Stello, allowed them to precisely measure the stellar parameters.

"We analysed the frequencies of standing sound waves inside the star to tell its size in the same way that you could tell the difference in size of a violin and cello by the difference in the pitch of the sound they produce," said Dr Stello.

This asteroseismic analyses showed that the radius of Kepler-37 is about 20% smaller than the Sun.

"Knowing the stellar radius is very important because the accuracy with which we can measure for the planet radius is limited by the uncertainty on the stellar radius," said Dr Stello.

"Our work from here is to keep working with the planet team at NASA to make seismic analysis of planet hosting stars, and there are some exiting results in the pipeline," Said Dr Stello.

Contact: Tom Gordon

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