Ben Shoemate's Notebook

By Christopher Dombrowski, Ars Technica

Seems like every week astronomers find a new exoplanet, one that’s the biggest or the smallest or the hottest or most habitable. However, this week astronomers are announcing a truly unique and new class of exoplanets: Jupiter sized planets that are in extremely large orbits or completely unbound from a host star altogether. And there appear to be a lot of them, as these planets seem to be more common than main sequence stars.

Finding a planet that is not associated with a star is no easy task. In the new search, a team of researchers used a technique called gravitational microlensing. As you look at a background field of stars, if an object passes between you and one of the stars, there will be a temporary brightening of that star. This occurs as the gravity of the object bends light around itself, which acts as a lens for light from the background star, hence “gravitational lensing.” Microlensing occurs when the foreground object is too small to create measurable distortion of the background star and only a brightening is observed. This makes it an ideal detector for small, dim objects.

The mass of the lensing object determines the duration of the brightening event — the longer the duration, the more massive. A Jupiter-sized object would produce lensing event with a duration of around one day.

The odds of a microlensing event occurring are exceedingly small, as the lensing object has to line up exactly between you and the background star. To compensate, astronomers looked at 50 millions of stars over several years, which yielded 474 microlensing events. Out of those 474, 10 had durations of less than two days, consistent with a Jupiter mass object.

No host stars were observed within 10 astronomical units of the lensing object. Previous work from The Gemini Planet Imager had set limits of the population of Jupiter-sized planets in extended orbits. From that data, the astronomers were able to estimate that 75 percent of their observed planets were most likely not bound to a host star at all, and are instead loose within the galaxy.

By creating a galactic-mass density model that takes into account this new class of object, astronomers were able to predict how many of these unbound planets there might be. They found that there are ~1.8 times as many unbound Jupiter-sized object as there are main sequence stars in our galaxy.

This raises a number of questions. Did these planets from near a star only to be ejected from the system? And if they truly have never been bound to any stars, do these planets represent a new planetary formation process? In any case, these observations have discovered a whole new population of Jupiter-sized planets in the Milky Way, and there are a lot of them.

I wonder if these new planets are like our Jupiter and, like our Jupiter, have moons which are geologically active and warm. If so, these new planets may have significantly increased the number of places that life may exist.

Image: NASA/JPL-Caltech [full-resolution image]

Citation: “Unbound or distant planetary mass population detected by gravitational microlensing.” The Microlensing Observations in Astrophysics (MOA) Collaboration and The Optical Gravitational Lensing Experiment (OGLE) Collaboration. Nature, Vol. 473, Pg. 349–352, 19 May 2011. DOI: 10.1038/nature10092

Source: Ars Technica

See Also:

• Ignored Exoplanet May Be a Watery World
• Exoplanet Hunter Finds Bounty of Multi-Planet Solar Systems
• Kepler Finds First Definitively Rocky Exoplanet
• Exclusive: Chat With Exoplanet Guru Geoff Marcy
• Odds of Finding Earth-Size Exoplanets Are 1-in-4
• Extragalactic Exoplanet Found Hiding Out in Milky Way