Giant planets do not come as lonely hearts

Scaled versions of Jupiter and Saturn orbiting a star 5000 light-years away, half as massive as the Sun, have been revealed from an effort involving a world-wide net of telescopes, including the UK's Liverpool Telescope on the Canary Islands. This marks the first discovery of another system of planets that has striking similarities with our Solar System. Moreover, it suggests that such giant planets do not favour the single-life but are more likely to be found in family groups. The research is published in the 15th February issue of Science.

Whilst there are more than 250 planets now known, there are only about 25 such systems with multiple planets and the newly discovered system resembles our own Solar System more closely than any previously observed.

Dr Martin Dominik, Royal Society University Research Fellow at the University of St Andrews, points out "Our gravitational microlensing technique is currently best suited for studying extra-solar planets that resemble the gas giants of the Solar System at their respective orbital radii, given that we do not need to wait for many years for them to complete their orbit."

The two newly discovered planets have revealed their existence through characteristic signatures in the received light during the gravitational microlensing event OGLE-2006-BLG-109. Rather than orbiting the observed star, these are associated with an unseen foreground star, systematically designated OGLE-2006-BLG-109L (where 'L' stands for 'lens), whose gravitational field (together with that of the planets) bent the light of the observed background star, with which it happened to be closely aligned.

While planet OGLE-2006-BLG-109Lb with 0.71 Jupiter masses is 2.3 times as far from its host star as the Earth is from the Sun, the less massive OGLE-2006-BLG-109Lc, 0.27 times the mass of Jupiter resides at twice the distance from its host star as its fellow companion.

Despite of the fact that their host star is only 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 between the two masses of the detected giant planets (close to 3:1) and the ratio between their orbital radii (1:2) are remarkably similar to those of Jupiter and Saturn. Similarly, the ratio between the orbital periods of 5 years and 14 years, respectively, resembles that between Jupiter and Saturn (2:5).

Due to the lack of respective signatures, additional gas-giant planets more massive than Saturn can only reside in orbits smaller than that of Venus or substantially wider than that of OGLE-2006-BLG-109Lc. This leaves the plausible possibility for terrestrial planets (like Mercury, Venus, Earth, and Mars) to reside 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, respectively, 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 they come as hierarchic systems rather than as lonesome 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 come 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 carry 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 can be expected to 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 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 on finding them and investigating whether these harbour any forms of life."

26 March 2006:

• the OGLE (Optical Gravitational Lens Experiment) team, led by Prof Andrzej Udalski from Warsaw University (Poland), gave notice of the event OGLE-2006-BLG-109 being in progress

28 March 2006:

• the OGLE team found an unexpected brightening by about 10 per cent, which could have been the signature of a planetary companion to the lens star

• other teams started follow-up observations on OGLE-2006-BLG-109.

  For the UK-based RoboNet microlensing programme, led by Prof Keith Horne at the University of St Andrews, Dr Martin Dominik from the same institution, devised a sampling strategy and passed it on to Dr Martin Burgdorf, the RoboNet project scientist at the Astrophysics Research Institute (ARI) of Liverpool John Moores University (LJMU), who entered the respective observing requests into the scheduler.

5 April 2006:

• OGLE-2006-BLG-109 shows a deviation from the predicted light curve.

• A preliminary model obtained within 12 hours by Dr Scott Gaudi, from Ohio State University, the lead author of the publication and member of the MicroFUN team, indicates a Jovian-class planet and predicts an additional peak on 8 April

6 April 2006:

• Surprisingly, a further peak was observed, which later turned out to be the signature of OGLE-2006-BLG-109Lb, the more massive planet being closer to the lens star

8 April 2006:

• The earlier predicted peak occurs, and confirms OGLE-2006-BLG-109Lc with a mass similar to that of Saturn

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 parent 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.