The Search for Extraterrestrial Civilizations

star field



Posted February 10, 2012

Exoplanets Found by Gravitational Lensing

An international team has used the technique of gravitational microlensing to measure how common planets are in the Milky Way. After a six-year search that surveyed millions of stars, the team concludes that planets around stars are the rule rather than the exception. The results appeared in Nature on 12 January 2012. “We used to think that the Earth might be unique in our galaxy. But now it seems that there are literally billions of planets with masses similar to Earth orbiting stars in the Milky Way,” concludes Daniel Kubas, co-lead author of the paper.

As the survey shows that our galaxy contains, on average, about 1.6 planets for every star, there is likely to be a minimum of 1,500 planets within just 50 light-years of Earth. The study concludes that there are far more Earth-sized planets than Jupiter-sized worlds. A rough estimate suggests more than 10 billion terrestrial planets in our galaxy

Over the past 16 years, astronomers have detected more than 700 confirmed exoplanets and over 2,000 planet candidates that have yet to be independently confirmed.  While the spectra and atmospheres of these worlds are starting to be probed, the basic question has remained: how commonplace are planets in the Milky Way?

Most currently known exoplanets were found either by detecting the effect of the gravitational pull of the planet on its host star (radial velocity method)or by detecting the planet as it passes in front of its star and slightly dims it (the transit method used by the Kepler telescope to detect over 2,000 planet candidates). Both of these techniques are more sensitive to planets that are either massive or close to their stars, or both. Gavitational microlensing can detect planets over a wider range of mass and further from their stars.

Arnaud Cassan, of the Institut d'Astrophysique de Paris, lead author of the Nature paper, explained: "Remarkably, these data show that planets are more common than stars in our galaxy. We also found that lighter planets, such as super-Earths or cool Neptunes, must be more common than heavier ones."

Wide-field survey campaigns such as OGLE (Optical Gravitational Lensing Experiment) and MOA (Microlensing Observations in Astrophysics) cover millions of stars every clear night in order to identify and report stellar microlensing events as early as possible. Follow-up collaborations, such as PLANET, monitor selected candidates more frequently, 24 hours a day, using a round-the-world network of telescopes.

The astronomers used observations, supplied by the PLANET and OGLE  teams, in which exoplanets are detected by the way that the gravitational field of their host stars, combined with that of possible planets, acts like a lens, magnifying the light of a background star. If the star that acts as a lens has a planet in orbit around it, the planet can make a transient detectable contribution to the brightening effect on the background star. The higher the mass of the "lensing" star, the longer is the duration of the microlensing event. Typical microlensing events due to a star last about a month. But the extra brightening due to a planet typically lasts a few hours to a couple of days.

The astronomers surveyed millions of stars looking for microlensing events. Only 3,247 such events were spotted between 2002 and 2007, as the precise alignment needed is very unlikely. Statistical results were inferred from detections and non-detections on a representative subset of 440 light curves.

Jean-Philippe Beaulieu (Institut d'Astrophysique de Paris), leader of the PLANET collaboration adds: "The PLANET collaboration was established to follow up promising microlensing events with a round-the-world network of telescopes located in the southern hemisphere, from Australia and South Africa to Chile. European Southern Observatory telescopes contributed greatly to these surveys.”

Microlensing is a very powerful tool, with the potential to detect exoplanets that could never be found any other way. But a very rare chance alignment of a background and lensing star is required for a microlensing event to be seen at all. And, to spot a planet during an event, an additional chance alignment of the planet’s orbit is also needed. In the six year's worth of microlensing data used in the analysis, three exoplanets were actually detected in the PLANET and OGLE searches: a super-Earth and planets with masses comparable to Neptune and Jupiter. By microlensing standards, this is an impressive haul. In detecting three planets, either the astronomers were incredibly lucky despite huge odds against them, or planets are so abundant in the Milky Way that it was almost inevitable.

The astronomers then combined information about the three positive exoplanet detections with seven additional detections from earlier work, as well as the huge numbers of non-detections in the six year's worth of data. The conclusion was that one in six of the stars studied hosts a planet of similar mass to Jupiter, half have Neptune-mass planets and two thirds have super-Earths. The survey was sensitive to planets between 75 million kilometers and 1.5 billion kilometers from their stars (in the Solar System this range would include all the planets from Venus to Saturn) and with masses ranging from five times the Earth up to ten times Jupiter.

Combining the results suggests strongly that the average number of planets around a star is greater than one. They are the rule rather than the exception.

"Results from the three main techniques of planet detection are rapidly converging to a common result: Not only are planets common in the galaxy, but there are more small planets than large ones," said Stephen Kane, a co-author from NASA's Exoplanet Science Institute at the California Institute of Technology, Pasadena, Calif. "This is encouraging news for investigations into habitable planets."

These results cover a different set of exoplanets explored by a gravitational-lens survey led by Takahiro Sumi of Osaka University in Japan, which estimated there are hundreds of billions of planets with orbits larger than Saturn's orbit, or free-floating throughout the galaxy.

Source: The Microlensing Observations in Astrophysics (MOA) Collaboration & The Optical Gravitational Lensing Experiment (OGLE) Collaboration. Nature Letter, 473, 349-352 (2011).