Planets may be vastly more numerous than believed


The Milky Way galaxy may be filled with millions upon millions of Jupiter-sized planets that have escaped their solar systems and are wandering freely in space, researchers said Wednesday in a finding that seems certain to make astronomers rethink their ideas about planetary formation.

Scientists had previously thought that about 20% of stars had massive planets attached to them, but the new results reported in the journal Nature suggest that there are at least twice as many planets as stars, and perhaps several times as many.

The finding “is a revelation in the sense that it looks like a quintupling of the number of gas giants in the universe,” said astronomer Alan Boss of the Carnegie Institution for Science, who was not involved in the research.


Scientists have long speculated that some planets may be vagabonds in interstellar space, but with these observations, “for the first time, we have fairly strong evidence that indeed this is happening,” said Caltech astronomer David Stevenson, who also was not involved in the research.

The discoverers of the wandering planets speculate that the orphan bodies were ejected from formative solar systems soon after they condensed from the interstellar dust that also formed the stars.

“If they are ejected, it’s a real puzzle as to how that happened because you have to be kicked out by something bigger than you,” Boss said.

The finding will have “theorists scratching their heads and sharpening their pencils for some time to come,” he added.

The orphan planets were discovered by an international team of astronomers headed by Takahiro Sumi of Osaka University in Japan. The team used the 5.9-foot telescope at Mount John University Observatory in New Zealand to regularly scan the innumerable stars at the center of the galaxy to search for so-called gravitational microlensing events.

When a massive object passes between the Earth and a distant star, the gravity of that object bends the light waves around it, acting like a magnifying lens to make the star appear brighter. How long the object remains brighter is a measure of the mass of the intervening body. Stars can cause the distant object to appear brighter for weeks, and Jupiter-sized objects produce a similar effect for days.


The Japanese team reported that it observed 10 Jupiter-sized objects, each about 10,000 to 20,000 light-years from Earth. Close examination of the data showed that there were no stars within a billion miles or so of the planets, about the distance from our sun to the outer solar system. The team therefore concluded that the objects are in orbits much larger than any previously observed or that they are wandering freely through interstellar space.

Their observations were independently confirmed by a second group of scientists using a 4.2-foot telescope in Chile.

Because researchers were observing only a very narrow slice of the sky — and because observing the objects requires a very precise alignment of star, object and Earth — they extrapolated that the objects are quite numerous, outnumbering the billions of stars in the galaxy by a ratio of at least 2 to 1.

The authors’ preferred explanation is that the objects were ejected from orbit around the stars where they formed. And if so many large objects were ejected, they concluded, then a much larger number of smaller, more easily ejected planets are also out there. Such planets, the size of Earth or smaller, would be very difficult to detect because they do not emit light and are too far from stars for any reflected light to be visible.

“The implications of this discovery are profound,” astronomer Joachim Wambsganss of the University of Heidelberg wrote in a commentary accompanying the study.

Researchers are now lobbying for the construction of an orbiting telescope dedicated to continuing the search for orphan planets.