More planets than stars in our Milky Way!

Artwork: ESO/M. Kornmesser

When you look at the night sky, from your favorite camping spot [dark sky site], its always amazing to see the myriad of stars painting the night sky.

According to a study released in the journal Nature tomorrow, there are actually more planets than stars in the Milky Way, our home galaxy. Come again? - That's right the ratio of planets to stars is thought to be greater than 1.

Most readers would be aware that the Kepler space mission has detected over 1600 exoplanet candidates so far, but over the past 16 years, astronomers have detected more than 700 (other) 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?

Arnaud Cassan (Institut dʼAstrophysique 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."


There are three methods for detecting exoplanets: 1)A spectroscopic radial velocity measurement 2)Direct observation of a transit where the planet moves in front of the parent star and 3)Microlensing.

Illustration: WASP 8b transits in a retrograde orbit - ESO/L. Calçada

According to the European Southern Observatory: "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".


Illustration: Microlensing lightcurve of EROS-BLG-2000-5 - ESO

So these events are more difficult to detect, but a very valuable detection method.

Jean-Philippe Beaulieu (Institut d'Astrophysique de Paris), leader of the PLANET collaboration [Probing Lensing Anomalies NETwork] 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.”

In the six year study the astronomers made three successful detections then combined the information 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 kilometres and 1.5 billion kilometres 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.


Artwork: ESO

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.

This result is consistent with some of the early results already coming out of the Kepler Program where already scientists are finding super earths more common than first thought. One of the core goals of the Kepler program is to determine the number of earth sized planets in our galaxy, so it will be interesting to see, as more data becomes available from Kepler, how these studies compare.

“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.


Image Credit: La Silla Observatory - ESO/Z. Bardon (www.bardon.cz)

More than half of the data from the PLANET survey used in this study came from the Danish 1.54-metre telescope at ESO's La Silla Observatory.

The year 2012 marks the 50th anniversary of the founding of the European Southern Observatory (ESO). ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive astronomical observatory.

So.... when you wish upon a star, you can also wish upon "1.n planets" as well ;-)

The calculation of the number of planets in the Milky Way continues!

Acknowledgements:
This research was presented in a paper, “One or more bound planets per Milky Way star from microlensing observations”, by A. Cassan et al., to appear in the 12 January issue of the journal Nature. The team is composed of A. Cassan (Institut dʼAstrophysique de Paris, France [IAP]; ESO), D. Kubas (IAP), J.-P. Beaulieu (IAP), M. Dominik (University of St Andrews, United Kingdom), K. Horne (University of St Andrews), J. Greenhill (University of Tasmania, Australia), J. Wambsganss (Heidelberg University, Germany), J. Menzies (South African Astronomical Observatory), A. Williams (Perth Observatory, Australia), U. G. Jørgensen (Niels Bohr Institute, Copenhagen, Denmark), A. Udalski (Warsaw University Observatory, Poland), M. D. Albrow (University of Canterbury, New Zealand), D. P. Bennett (University of Notre Dame, Notre Dame, USA), V. Batista (IAP), S. Brillant (ESO), J. A. R. Caldwell (McDonald Observatory, Fort Davis, USA), A. Cole (University of Tasmania), Ch. Coutures (IAP), K. Cook (Lawrence Livermore National Laboratory, USA), S. Dieters (University of Tasmania), D. Dominis Prester (University of Rijeka, Croatia), J. Donatowicz (Technical University of Vienna, Austria), P. Fouqué (Université de Toulouse, France), K. Hill (University of Tasmania), N. Kains (ESO), S. Kane (NASA Exoplanet Science Institute, Caltech, USA), J.-B. Marquette (IAP), K. R. Pollard (University of Canterbury, New Zealand), K. C. Sahu (STScI, Baltimore, USA), C. Vinter (Niels Bohr Institute), D. Warren (University of Tasmania), B. Watson (University of Tasmania), M. Zub (Heidelberg University), T. Sumi (Nagoya University, Japan), M. K. Szymański (Warsaw University Observatory), M. Kubiak (Warsaw University Observatory), R. Poleski (Warsaw University Observatory), I. Soszynski (Warsaw University Observatory), K. Ulaczyk (Warsaw University Observatory), G. Pietrzyński (Warsaw University Observatory), Ł. Wyrzykowski (Warsaw University Observatory).