News & Events

Giant Planets are not Lonely Hearts

14th February 2008

Artist's impression of newly discovered planet

(1) Artist's impression of newly discovered planets OGLE-2006-BLG-109Lb and OGLE-2006-BLG-109Lc together with their host star OGLE-2006-BLG-109L. Credit KASI/CBNU/ARCSEC.

Image above also available as a 12 sec avi movie (1.5MB). Credit KASI/CBNU/ARCSEC.

(2) Artist's impression of planets OGLE-2006-BLG-109Lb and OGLE-2006-BLG-109Lc in their orbits around their host star OGLE-2006-BLG-109 with speculative further inner terrestrial planets added. Credit KASI/CBNU/ARCSEC.

KASI: Korea Astronomy and Space Science Institute
CBNU: Chungbuk National University
ARCSEC: Astrophysical Research Center for the Structure and Evolution of the Cosmos

Two gas giant planets, similar to Jupiter and Saturn, have been discovered orbiting a star 5000 light-years away in a planetary system with striking similarities to our own Solar System. The discovery suggests that giant planets do not live alone but are more likely to be found in family groups. The planets were discovered using a world-wide network of telescopes, including the UK's Liverpool Telescope on the Canary Islands. The research is published in the 15th February issue of Science.

Whilst there are more than 250 planets now known to be orbiting other stars, there are only about 25 such systems with multiple planets. The newly discovered system resembles our own Solar System more closely than any previously observed.

The discovery relied upon a chance alignment of the star, named OGLE-2006-BLG-109L, and its orbiting planets with a background star. Light from the more distant star is focused by the gravity of the star and planets causing a characteristic brightening called gravitational microlensing from which the properties of the planets can be deduced.

Dr Martin Dominik, Royal Society University Research Fellow at the University of St Andrews, points out "Our gravitational microlensing method is ideally suited for discovering gas giant planets that resemble those in our own Solar System. Other techniques require waiting many years for the planets to complete orbits around their host stars."

The mass of planet OGLE-2006-BLG-109Lb is 0.71 times that of Jupiter and it lies 2.3 times as far from its host star as the Earth is from the Sun. The second planet, OGLE-2006-BLG-109Lc, is less massive, 0.27 times the mass of Jupiter, and twice as far away from the host star.

Despite their host star only being half as massive as the Sun, and therefore cooler, the OGLE-2006-BLG-109L planetary system otherwise bears a remarkable similarity to our Solar System. Both the ratio of the masses of the two giant planets (close to 3:1) and the ratio of their distances from the host star (1:2) are remarkably similar to those of Jupiter and Saturn. The ratio between the orbital periods of 5 years and 14 years, respectively, also closely resembles that between Jupiter and Saturn (2:5).

More hidden planets?

These observations show that if there are any additional gas-giant planets more massive than Saturn in this new planetary system, then they can only be in orbits either very close to the host star (closer than that of Venus to our Sun) or very far away (substantially wider than that of OGLE-2006-BLG-109Lc). This leaves open the possibility that terrestrial planets (like Mercury, Venus, Earth, and Mars) lie inside the orbit of OGLE-2006-109Lb, which is likely to be the innermost giant planet. Moreover, planets taking the roles of Uranus and Neptune could also be present. These features make the OGLE-2006-BLG-109L system the most similar to the Solar System amongst the about 25 exo-planetary systems discovered so far.

Given that we already know from observations that not all stars host gas-giant planets, and that the detection of each of them is not guaranteed, the double catch around OGLE-2006-BLG-109L suggests that giant planets come as hierarchical systems rather than as single objects. Dr Nicholas Rattenbury, STFC-funded Postdoctoral Research Associate at the Jodrell Bank Centre of Astrophysics, points out: "Like humans, gas-giant planets appear to prefer not to exist as lonely hearts."

The contributing observations with the Liverpool Telescope (LT) were carried out as part of the RoboNet microlensing programme, whose principal investigator, Prof Keith Horne from the University of St Andrews remarks "The flexible scheduling and short response time of robotic telescopes is ideally suited to carrying out a time-critical programme like the search for extra solar planets by microlensing." Apart from the Liverpool telescope, RoboNet exploits two further identical robotic telescopes with a diameter of 2m, the largest of their kind.

The RoboNet microlensing programme is spread over the network by means of intelligent-agent technology built by the eSTAR (e-Science Telescopes for Astronomical Research) Project. Dr Alasdair Allan from the University of Exeter, one of the core developers of eSTAR, explains "Single isolated telescopes are rapidly being integrated into expanding smart telescope networks, spanning continents and responding to transient events in seconds. These time-critical observations can be optimally scheduled across the network using Intelligent Agent technology which negotiates a contract for the observations with the remote telescopes".

The UK microlensing planet hunters are now preparing to boost their capabilities by adopting a fully-automated three-step approach of survey, follow-up, and anomaly monitoring. Enabled by the ARTEMiS (Automated Robotic Terrestrial Exoplanet Microlensing Search) expert system that determines the optimal target to be followed at any given time for any observing site, ground-based observations with a global network of telescopes could not only lead to the first detection of an Earth-mass extra-solar planet, but even of less massive ones.

Dr Dominik concludes: "While most planetary systems around other stars substantially differ from the Solar system, a series of recent detections have brought us closer and closer to home. Sooner rather than later, someone will discover an Earth-mass planet orbiting a star other than the Sun - and it could be us."

Prof Horne adds, "Apart from individual spectacular discoveries, the technique of gravitational microlensing allows us to infer a census of planets within the Milky Way. Once we know that planets similar to Earth are common, it is straightforward to go ahead and find them and eventually to investigate whether they harbour any forms of life."

Timeline of a global discovery

26 March 2006:

28 March 2006:

5 April 2006:

6 April 2006:

8 April 2006:

Summary of Planet Properties

OGLE-2006-BLG-109Lb OGLE-2006-BLG-109Lc Jupiter Saturn
Mass 0.71 Jupiter mass 0.27 Jupiter mass 1 Jupiter mass 0.30 Jupiter mass
Orbital distance from host star 2.3 AU 4.6 AU 5.2 AU 9.6 AU
Orbital period 5 years 14 years 12 Years 30 Years

Where AU = Astronomical Unit, the distance from the Earth to the Sun.

Notes for Editors

Microlensing technique

The foundation of the technique of gravitational microlensing dates back as far as 1912, when Albert Einstein predicted that a star could exhibit a transient brightening caused by the deflection of its light due to the gravitational field of an intervening foreground star, thereby acting as a 'gravitational lens'.

Monitoring hundreds of millions of stars each night for such a rare phenomenon by the Polish-US OGLE (Optical Gravitational Lensing Experiment) and the New Zealand-Japan MOA (Microlensing Observation in Astrophysics) collaborations provides the scientific community with nearly a thousand microlensing events per year, which are alerted in real time.

While microlensing events on stars themselves last about a month, a planet orbiting the (foreground) lens star can create an additional small dip or blip, lasting between several hours and several days, depending on whether the mass of the planet resembles that of Earth or of Jupiter.

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