Imagine a planet where the night sky is all you have, any time, anywhere you go – where the phrase “day job” has no meaning.
The Milky Way may hold a vast number of such sunless planets – perhaps up to 100,000 times more than all the stars in the galaxy, according to a new estimate from researchers in the United States and Britain.
Since 1995, when hunting for planets outside the solar system grew from a fringe field to mainstream astronomy, researchers have uncovered as many as 760 extrasolar planets orbiting other stars – as the Earth orbits the sun – with more than 2,000 additional candidates awaiting confirmation.
The vast majority of these are planets in the classical sense – meaning they orbit host stars, although often arrayed in unusual patterns compared with Earth's home system.
Since 2000, however, astronomers have discovered planets with no obvious stellar home. A group of Spanish astronomers reported that year discovering planets ranging from five to 15 times Jupiter's mass free floating in a cluster of young stars in the constellation Orion. Last year, two groups of astronomers jointly announced the discovery of 10 Jupiter-class planets, the vast majority free of the grip of any host star. The results appeared last May in the journal Nature.
With evidence mounting that these planetary nomads are no observational fluke, “we were interested in asking the question: Is this just the tip of the iceberg?” says Louis Strigari, a researcher at the Kavli Institute for Particle Astrophysics and Cosmology at Stanford University in Palo Alto, Calif., who led the team making the calculation.
“We were curious about how many of these types of objects could exist,” Dr. Strigari says. The team based its estimate on what researchers have gleaned about the amount of visible matter in the galaxy as well as the abundance of existing chemical elements heavier than hydrogen and helium – the primordial elements from which the universe evolved following the big bang some 13.8 billion years ago. These heavier elements, forged in thermonuclear furnaces deep in the hearts of stars, provide the building blocks for the gases, ice, and rock that comprise planets.
The team's estimate covers planets with masses ranging from Pluto-like to planets with several times Jupiter's mass.
Stigari cautions that the calculation represents a rough cut at a maximum number of nomads. Too little information is known about these kinds of planets and their formation to allow for more-precise estimates. The estimate has been submitted for publication in the Monthly Notices of the Royal Astronomical Society in Britain.
The ratio of nomads to stars “is a staggering number, if true,” says Ray Jayawardhana, an astronomer at the University of Toronto who studies the evolution of planets.
“But we have no evidence at all from observations yet that anything close to that number is really out there,” he says. The most one can say from the detection of free-floating planets in clusters of young stars, which typically are several times Jupiter's mass is that the ratio of wanderers to stars is well below one to one, he adds.
But, he notes, the team's estimate also covers far smaller free-floaters, including mass ranges for which there are yet no observed nomads.
Where do homeless planets come from?
“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.