News & Events


14th March 2006

In a paper submitted this week to the journal Astrophysical Journal Letters, an international collaboration of astronomers report the discovery of a Neptune-sized planet orbiting a distant star.

The new planet has a mass roughly 13 times that of the Earth, and orbits its parent star at a distance of approximately 3 times the distance from the Earth to the Sun. The planet is likely to be very similar to Neptune: a cold, icy world with a surface temperature of about -200 degrees Celsius. The planetary system is distant, about 9000 light years away.

Most known extra-solar planets are massive planets similar to Jupiter. Finding lighter planets such as the new Neptune-mass planet, is more difficult. Discovering with masses similar to Earth's, is harder still. However, discovering planets with masses approaching that of Earth is possible. The only technique currently available that allows the detection of terrestrial mass planets around normal stars is called gravitational microlensing.

The technique uses the gravitational fields of stars as naturally occurring lenses. Microlensing occurs when a massive object, such as a star, almost exactly lines up with observers on Earth and a background star. The light from the background star is deflected by the gravity of the intervening object. Analysing how the magnified light from the background star changes allows the detection of planets orbiting the foreground star.

Gravitational microlensing was first proposed as a method for detecting extra-solar planets by Dr Shude Mao (Jodrell Bank Observatory) and Prof Bohdan Paczynski (Princeton) in 1991. The first confirmed detection of an extra-solar planet via microlensing was made in 2004. Dr Nicholas Rattenbury (Jodrell Bank Observatory) was one of the lead researchers in this discovery, working at the time at The University of Auckland, New Zealand.

The recent planet discovery was made possible through the survey observations of the Polish/US group OGLE, which observe many fields toward the centre of our Galaxy every night, searching for microlensing events. One event started to show a microlensing event where the background source was magnified about 800 times. Such events were predicted by Dr Rattenbury and colleagues from New Zealand in 2002 to be very sensitive to low mass planets.

Noting the rapid increase in amplification, Dr Andrzej Udalski (OGLE, Warsaw University) issued an alert, recommending additional observations to be made. Dr Andy Gould (Ohio State University, MicroFUN collaboration) requested immediate observations from Kitt Peak, Arizona. Observations from other telescopes in Auckland, New Zealand and Chile (MICROFUN collaboration) and Hawaii (RoboNET) were similarly obtained.

The data were independently analysed by researchers in the UK, USA, Chile and NZ. Dr Rattenbury was the first to identify a class of models that fitted the observational data and corresponded to a low-mass planet. Subsequent analysis by Dr Rattenbury and collaborators confirmed that the planetary signal was real, and the planet was approximately the mass of Neptune.

A preprint of the paper submitted to The Astrophysical Journal appears at


Dr. Nicholas Rattenbury
Jodrell Bank Observatory,
University of Manchester,
Cheshire, UK
Tel +44 (0)1477 572653