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Could free-floating 'nomad' planets carry seeds of life in the universe?

A 'nomad' planet of the right mass, with the right atmosphere, and some source of heat – perhaps radioactive decay or tectonic activity – could allow for life either on the surface or underground.

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Where do homeless planets come from?

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“We don't have a good theory for how such objects form based on observations,” Dr. Stigari acknowledges.

Some could form via the classical route – emerging from a disk of dust and gas encircling a young host star. Then they face ejection through gravitational interactions with other planets in the system.

Modeling suggests that if two Jupiters form in the same system, one stands a good chance of getting ejected, Strigari says. Indeed, some observational evidence suggests that for each star in Milky Way, there are two such nomads.

“At this stage, that is confounding theorists,” Stiragi says, because it's unclear how the estimated number of solar systems in the galaxy could produce enough Jupiters to trigger enough ejections to yield the 2-to-1 ratio."

On the other hand, some nomads may never carry the stigma of ejection because they may have formed directly from the gravitational collapse of a portion of an interstellar cloud, a process that also gives rise to stars. With planet-mass objects, it could be a case of arrested development.

In fact, both could be at work, Dr. Jayawardhana notes

Based on his team's observations of free-floating planets in young, star-forming regions in the Milky Way, planets with about five times Jupiter's mass or greater likely formed from a process similar to that of stars. Such planets appear to have their own mini disks of dust and gas orbiting them, just as stars do. For planets with masses comparable to Jupiter's or less, they likely formed within a star's protoplanetary disk, he says.

Either way, the potentially enormous number of nomads may play a role in the spread of life throughout the galaxy, the team suggests. A planet of the right mass, with the right atmosphere, and some source of heat – perhaps radioactive decay or tectonic activity – could allow for habitats either on the surface or underground. An underground hideaway might be the best place, with no stellar magnetic field to act as an initial barrier to galactic cosmic rays.

Collisions with other objects might knock off bacteria-laden chunks that could be available to land on some more-hospitable planet.

Whatever the prospect, the hunt for these wanderers is likely to heat up. Astronomers have found many of them using a technique known as gravitational microlensing. In essence researchers detect planets by tracking the brightness of the system as it passes in front of another star. Gravity from the foreground star magnifies the light from the star behind it, brightening the light astronomers record. The presence of a planet adds some extra flare, which appears either before or after the host star itself triggers brief brightening.

Although hunting for such gravitational microlenses requires patience, the approach can detect planets at much greater distances from Earth than other planet-hunting techniques. This provides a more representative sample for estimating the galaxy's population of planets than other approaches, Stiragi says. Other techniques generally are limited to detecting planets orbiting stars relatively close to our solar system,

NASA and the European Space Agency are both pursuing space-telescope projects that will allow astronomers to monitor more of the sky for such lensing events. In addition, researchers in the US are building a ground-based telescope – the Large Synoptic Survey Telescope – that will image the entire night sky twice a week.

The hope is that by returning frequently to the same patch of sky, the telescope will be able to pick up sudden changes to objects – such as the brightening of gravitational microlenses – that others can quickly train ground and space-based telescopes to study in more detail.

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