News & Events

Discovery of the smallest yet Earth-like planet

25th January 2006

UK astronomers have played a significant role in the discovery of a new extra solar planet - an exoplanet - that is significantly more Earth-like than any of the other 160 exoplanets found so far. The discovery of the new planet, which is only about 5-times as massive as the Earth making it the least massive exoplanet ever found orbiting an ordinary star, marks a groundbreaking result in the search for Earth-like planets capable of supporting life. The results are published in the 26th January edition of Nature.

Artist's impression of the new planet.
An artist's impression of the new planet.

The planet's parent star is a red dwarf, roughly 5 times less massive than our Sun, and is located around 25,000 light years from Earth close to the centre of our own galaxy, the Milky Way. The newly-discovered planet, designated OGLE-2005-BLG-390Lb, has an orbital period of 10 years and is about three times as far from its parent star as the Earth is from our Sun.

Its relatively cool parent star and large orbit implies that the likely surface temperature of the planet is 220 degrees Centigrade below zero, far too cold for liquid water which would be a requirement for the development of life. It is likely to have a thin atmosphere, like Earth, but its rocky surface is probably buried deep beneath frozen oceans. It may therefore more closely resemble a more massive version on Pluto, rather than the inner rocky planets like Earth and Venus.

Contrary to most exoplanets discovered so far [about 160] the new planet was found using the 'microlensing' technique, based on an effect predicted by Albert Einstein in 1915.

With the microlensing method, the gravity of a dim, intervening star act as a giant natural telescope, magnifying a more distant star, which then temporarily looks brighter. A small 'defect' in the brightening reveals the existence of a planet around the lens star. The planet is not directly 'seen', or even the star that it's orbiting, but its presence can be deduced from theeffect of their gravity.

Such an intervening star causes a characteristic brightening that lasts about a month. Any planets orbiting this star can produce an additional signal, lasting days for giant planets down to hours for Earth-mass planets.

Discovery light curve
The microlensing event light curve

Prof Keith Horne (University of St Andrews), who is leading the RoboNet microlensing planet search, one of the microlensing campaigns involved in the collaborative discovery, concludes that "Microlensing is the fastest way to find small cool planets, down to the mass of the Earth. Our first three planet discoveries indicate that small cool planets are abundant. If we can deploy robotic telescopes at additional sites in the southern hemisphere, we can expect to find several more cool planets every year, which could include the first detection of extra-solar Earths."


The discovery is the joint effort of three independent microlensing campaigns: PLANET/RoboNet, OGLE, and MOA, involving a total of 73 collaborators affiliated with 32 institutions in 12 countries (France, United Kingdom, Poland, Denmark, Germany, Austria, Chile, Australia, New Zealand, United States of America, South Africa, and Japan).

Dr Martin Dominik (University of St Andrews), who is the co-leader of PLANET/RoboNet said, "We first saw the usual brightening reaching a peak magnification on 31 July 2005, after which the event started to fade back symmetrically. On 10 August, however, there was a small 'flash' lasting about half a day. By succeeding in catching this anomaly with two of the telescopes of our network and with careful monitoring, we were able to conclude that the lens star is accompanied by a low-mass planet."

Dr Nicholas Rattenbury (Jodrell Bank Observatory), member of the MOA collaboration, points out "The chance that an observed background star is sufficiently magnified at a given time is only about one in a million. However, with more than 100 million stars routinely monitored by the OGLE and MOA surveys, there are about 120 ongoing events at any one time where the lens star can be probed for surrounding planets ' leading to the prospect of seeing many more low mass objects."

In order to catch and characterize a planet, nearly-continuous round-the-clock high-precision monitoring of ongoing microlensing events is required, which is achieved by the PLANET network of 1m-class telescopes consisting of the ESO 1.54m Danish at La Silla (Chile), the Canopus Observatory 1.0m (Hobart, Tasmania, Australia), the Perth 0.6m (Bickley, Western Australia), the Boyden 1.5m (South Africa), and the SAAO 1.0m (Sutherland, South Africa). PLANET operates a common campaign with RoboNet, the UK operated network of 2m-class, fully robotic telescopes currently comprising the Liverpool Telescope [Roque de Los Muchachos, La Palma, Spain] and the Faulkes Telescope North [Haleakala, Hawaii].

As Prof Michael Bode (Liverpool John Moores University), principal investigator of the RoboNet project, states "With two main scientific aims, namely the identification of the origin of Gamma-Ray Bursts, and the microlensing census of extra-solar planets, the RoboNet telescopes provide automatically-scheduled, on-demand observations with the required crucial short response times."

Notes for Editors

Microlensing is more sensitive to cooler planets and to more distant and lower-mass parent stars than other techniques. OGLE-2005-BLG-390Lb is only the third extra-solar planet resulting so far from microlensing searches, from a total of more than 160 now known.

The other two microlensing planets have masses of a few times that of Jupiter, where the detection of one of them, OGLE-2005-BLG-071Lb, involved PLANET/RoboNet data taken in April 2005. The low number of detections for planets of Jupiter-mass or above, despite being easier to detect, indicates that gas giant planets are rare around red dwarfs. All three detected planets have red dwarf parent stars, which are very dim stars but also very common and therefore favoured by the microlensing searches that rely on the gravity, rather than the light, of the lens star.

The discovery of a sub-Neptune mass object already as the third one detected via microlensing is a strong hint that these objects in contrast are quite common. In fact, "core-accretion" planet-formation simulations predict a large fractional abundance of planets with masses below 10 Earth masses at orbits between 1 AU and 10 AU. By coincidence, these orbital separations match well the range preferred by microlensing, making it an ideal technique for studying this population down to Earth mass.


PLANET (Probing Lensing Anomalies Network):



Dr. Martin Dominik
SUPA St Andrews
School of Physics & Astronomy
Phone: 	+44-1334-463066 (office)
        +44-1334-470305 (home)

Prof. Keith Horne
SUPA St Andrews
School of Physics & Astronomy
Phone: 	+44-1334-463322 (office)

Prof. Michael Bode
Astrophysics Research Institute
Liverpool John Moores University
Phone: +44-151-231-2920/2919 (office)
       +44-7968-422360 (mobile)

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

Julia Maddock
PPARC Press Office
Tel +44 1793 442094 
Mobile +44 7901 514975

The Particle Physics and Astronomy Research Council (PPARC) is the UK’s strategic science investment agency. It funds research, education and public understanding in four broad areas of science - particle physics, astronomy, cosmology and space science.

PPARC is government funded and provides research grants and studentships to scientists in British universities, gives researchers access to world-class facilities and funds the UK membership of international bodies such as the European Organisation for Nuclear Research, CERN, the European Space Agency and the European Southern Observatory. It also contributes money for the UK telescopes overseas on La Palma, Hawaii, Australia and in Chile, the UK Astronomy Technology Centre at the Royal Observatory, Edinburgh and the MERLIN/VLBI National Facility.