Plentiful Planet Population

Planets around stars are the rule rather than the exception

Astrobiologist Ron Greeley
This artists’s cartoon view gives an impression of how common planets are around the stars in the Milky Way. The planets, their orbits and their host stars are all vastly magnified compared to their real separations. A six-year search that surveyed millions of stars using the microlensing technique concluded that planets around stars are the rule rather than the exception. The average number of planets per star is greater than one. Credit: ESO/M. Kornmesser

An international team, including three astronomers from the European
Southern Observatory (ESO), has used the technique of gravitational
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 appear in the journal Nature on 12 January 2012.

Over the past 16 years, astronomers have detected more than 700 confirmed exoplanets and have started to probe the spectra and atmospheres of these worlds. While studying the properties of individual exoplanets is undeniably valuable, a much more basic question remains: 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 or by
catching the planet as it passes in front of its star and slightly
dims it. Both of these techniques are much more sensitive to planets
that are either massive or close to their stars, or both, and many
planets will be missed.

An international team of astronomers has searched for exoplanets using
a totally different method — gravitational microlensing — that can
detect planets over a wide range of mass and those that lie much
further from their stars.

Arnaud Cassan (Institut d’Astrophysique de Paris), lead author of the
Nature paper, explains: “We have searched for evidence for exoplanets
in six years of microlensing observations. 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.”

Astrobiologist Ron Greeley
The Milky Way above the dome of the Danish 1.54-metre telescope at ESO’s La Silla Observatory in Chile. The central part of the Milky Way is visible behind the dome of the ESO 3.6-metre telescope in the distance. On the right the Magellanic Clouds can be seen. This telescope was a major contributor to the PLANET project to search for exoplanets using microlensing. The picture was taken using a normal digital camera with a total exposure time of 15 minutes. Credit: ESO/Z. Bardon ( (

The astronomers used observations, supplied by the Probing Lensing Anomalies NETwork (PLANET) and Optical Gravitational Lensing Experiment (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 detectable contribution to the
brightening effect on the background star.

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. ESO 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

Although for these reasons finding a planet by microlensing is far
from an easy task, 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 and had hit the jackpot despite huge odds
against them, or planets are so abundant in the Milky Way that it was
almost inevitable.

Artist´s conception of our Milky Way galaxy, with the location of our star, the Sun, noted. Could planets be common around the stars in our home galaxy? Image Credit: NASA

A super-Earth has a mass between two and ten times that of the
Earth. So far 12 microlensing planets have been published in total,
using various observational strategies.

The astronomers 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 — non-detections are just as important for the
statistical analysis and are much more numerous. 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

The astronomers surveyed millions of stars looking for
microlensing events. Only 3,247 such events in 2002-2007 were spotted
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.

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. “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.