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RSAA News of the Month: January 2006

At Last! A Cool, Rocky Planet.
International Microlensing project discovers a small planet

An international team of 73 astronomers from 32 institutions in 12 countries has found a small, cool planet orbiting a star in the inner Milky Way. Australian members of the team are RSAA Director Prof Penny Sackett, Drs Andrew Williams and Ralph Martin of Perth Observatory, and Drs John Greenhill, Kym Hill and Stefan Dieters of the University of Tasmania.

Australian telescopes involved in the discovery of OGLE-2005-BLG-390Lb. On the left, the 1-m telescope of the Canopus Hill Observatory, near Hobart, Tasmania. On the right, the 61 cm telescope of the Perth Observatory at Bickley, Western Australia.

Weighing in at around 5 times the mass of Earth, this planet, OGLE-2005-BLG-390Lb, is the least massive discovered so far. It orbits its parent star at a distance 3 times the distance of Earth from the Sun (3 AU), and takes 11 years for one orbit. The parent star is much smaller than the Sun, only 0.2 times as massive which means that it emits much less heat than than the Sun. This puts the surface temperature of the newly-discovered planet at a chilly minus 225 degrees Celsius. The star is about 23,000 light-years away from us.

The low mass and temperature mean that this is a planet made of rock and ice, quite unlike the "hot Jupiters" - large gaseous planets orbiting close to their parent star - which make up most of the extra-Solar planets discovered so far. Its discovery marks a groundbreaking result in the search for planets that may support life.

Even more exciting is that the type of star involved is most likely an M dwarf star, the most common type of star in our Milky Way galaxy. Current theory predicts that planets forming around such stars should be Earth-size to Neptune-size and in orbits less than 10 AU from the star. OGLE-2005-BLG-390Lb is "right in the box" and so provides strong support for the theory, and indicates that Earth-sized planets should be common in the Milky Way.

Professor Penny Sackett, co-founder of the PLANET team and its principal investigator for several years said, "This amazing discovery required worldwide cooperation and teamwork. It's delightful to see the crucial role played by the original Australian team. This is a tremendously exciting discovery which will shed much information about extra-solar planets in our galaxy and provides tantalising information about the potential of our galaxy to house another Earth-like planet where conditions are conducive to some form of life. We’re getting closer to discovering an Earth-twin.”

Most discoveries of extra-Solar planets are made by observing the "wobble" of the parent star caused by orbiting planets (the Doppler method), or by measuring the change in light as planets pass in front of the star (the transit method). These methods work best for detecting large planets close to their star. OGLE-2005-BLG-390Lb was found using a different method, microlensing, which has the potential to detect much smaller planets over a large range of orbital distances.

Extra-solar planets and methods of discovery.
The graph shows the masses, orbital periods and orbital radii for the 169 exoplanets known in September 2005, and the limits of the techniques used to detect them. The left-hand axis gives the mass compared to the mass of the Sun; the axis is logarithmic, so -2 = 1/100th the mass of the Sun, -6 = 1/1,000,000th the Solar mass. The bottom axis is the radius of the orbit compared to that of Earth (1 AU is the Earth-Sun distance) and the top axis is the orbital period in days or years. Black letters show the position of the Solar System planets.
The 158 black marks are planets discovered by the Doppler method. The black lines mark the limits of detection using this method on various telescopes, the upper line being the limit for ground-based instruments and the lower line being that for space-based ones. The planets found by this method are all much more massive than Earth.
The 8 blue dots are planets discovered by the Transit method. The blue lines are the detection limits for ground-based and space-based instruments. These planets are all massive and close to their parent star.
The 3 red markers are planets found by Microlensing. The red lines mark the detection limits. OGLE-2005-BLG-390Lb (circled) is the most Earth-like of the 169 in mass, having a mass less than Uranus or Neptune.
Green lines show the detection limits for astrometric detection. So far, no extra-solar planet has been found by this technique.
image: Keith Horne

Microlensing occurs when a star passes in front of a background star. The gravitational field of the foreground star acts as a lens, bending and focussing the light of the background star. As the foreground star passes across, the light from the background star brightens and then fades in a very characteristic manner. Planets orbiting the star produce secondary bumps on the light curve. Anaylsis of the light curve gives the planet-to-star mass ratio, the planet-star separation and the geometry of the lensing system.

(left) Image of the crowded star field where the lensing event occured.
(right) Diagram of the geometry of the event. The lensing star and planet are shown in blue and the background star in orange. Relative movement of the background star is marked by the yellow line. The blue diamond-shaped spot is the "caustic", the region of gravitational magnification for this system. The position of the caustic depends on the distances and masses of the stars involved and their separation across the line of sight. In reality, the separation of the two stars is so small that they are effectively superimposed and the light curve obtained is the sum of their brightnesses and the magnification produced by the lensing.
images: Daniel Kubas

The start of the OGLE-2005-BLG-390 event was recorded by the Optical Gravitational Lensing Experiment (OGLE) team on 11 July 2005. They immediately alerted other teams, PLANET (Probing Lensing Anomolies NETwork)and MOA (Microlensing Observations in Astrophysics). This produced a world-wide network capable of keeping the event under constant observation, with telescopes in Chile, New Zealand, Australia, South Africa, Hawaii and the Canary Islands. On 10 August the tell-tale planetary bump appeared. Observations from Perth Observatory and Canopus Observatory (Tasmania) were vital in tying down the peak and the shape of both the stellar and the planetary curves.

Light curve of microlensing event OGLE-2005-BLG-390
Data obtained by the PLANET/RoboNet, OGLE, and MOA teams of the microlensing event OGLE-2005-BLG-390, showing the planetary deviation on its falling part, lasting about a day. The data are colour-coded in order to indicate the telescope obtaining the data: Danish 1.54m at ESO La Silla (Chile), Perth 0.6m (Bickley, Western Australia), Canopus 1.0m (Hobart, Tasmania, Australia), Faulkes North 2.0m (Halakela, Hawaii, USA), Warsaw 1.3m (Las Campanas, Chile), and MOA 1.8m at Mt John Observatory (New Zealand).

Many of these scientists have been working together for years, studying planets, stellar atmospheres and dark matter. The geographical location of Australia allowed Australian researchers to record key data for this record-breaking event.

PLANET team leader, Jean-Philippe Beaulieu, sums up: "The discovery of a sub-Neptune mass planet encourages the intensification of microlensing planet searches with current and additional facilities from the ground - or even with a space-based campaign in the near feature - by providing an observational hint that further low-mass planets will be detected and that it is a straight path to the discovery of a twin Earth".

The discovery was reported in the January 26, 2006, edition of Nature .

The ANU press release can be read here, and the ESO release here.

The Perth observatory web page on the event is here.

For information on the teams: click PLANET, RoboNet, OGLE, MOA

For previous Monthly News items, click here.