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'Nomad' planets roam Milky Way without stars

Researchers estimate planets that don't orbit a sun could greatly outnumber the stars in the Milky Way and speculate that these nomad planets could harbor bacterial life.

An artist's rendition of a nomad planet. Greg Stewart /SLAC National Accelerator Laboratory at Stanford

In the search for planets, astronomers have traditionally hunted around stars. But a new study estimates that planets that don't orbit stars could be far more plentiful than previously thought.

The Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) at Stanford reported yesterday that new methods have dramatically raised the estimate of "nomad planets" in the Milky Way. There could be 100,000 more free-floating planets than stars in the galaxy.

Though they don't have the sun's energy to support life, some of these planets could have the conditions to support--and spread--microbial life through a combination of a thick atmosphere and heat generated by radioactive decay from the planet itself.

"If any of these nomad planets are big enough to have a thick atmosphere, they could have trapped enough heat for bacterial life to exist," Louis Strigari, who led research on the project, said in a statement. His team published a paper in the Monthly Notices of the Royal Astronomical Society on the new estimate.

A technique called gravitational lensing, which measures changes in light from the gravity of planets, has identified hundreds of sun-orbiting stars in the galaxy.

The team at Stanford deduced the number of nomad planets with complex math. Based on the gravitational pull of the Milky Way and the amount of matter available to objects in space, they could estimate matter available for planet-size objects, according to Stanford.

The group is hoping to confirm its estimate for nomad planets in the early 2020s, when more-accurate telescopes detect small objects in space.

"What is wonderful is that we can now start to address this question (of life on other planets) quantitatively by seeking more of these erstwhile planets and asteroids wandering through interstellar space, and then speculate about hitchhiking bugs," co-author and KIPAC Director Roger Blandford said in a statement.