Study says Rogue Worlds May Outnumber Planets with Suns

by

Astronomers have discovered a vast population of Jupiter-mass worlds that float through space without any discernible host star, a new study finds.

While some of these exoplanets could potentially be orbiting a star from very far away, the majority of them most likely have no parent star at all, scientists say. And these strange worlds are not mere statistical anomalies. They likely outnumber “normal” alien planets with obvious parent stars by at least 50 percent, and they are nearly twice as common in our galaxy as main-sequence stars, according to the new study.

Astronomers have long predicted the existence of free-flying “rogue alien planets.” But their apparent huge numbers may surprise many researchers, and could force some to rethink how the planets came to be.

“Previous observations of bound planets tell us only about planets which are surviving in orbits now,” said study lead author Takahiro Sumi, of Osaka University in Japan. “However, [these] findings inform us how many planets have formed and scattered out.”

Sumi and his colleagues made the find using a method called gravitational microlensing, which watches what happens when a massive object passes in front of a star from our perspective on Earth. The nearby object bends and magnifies the light from the distant star, acting like a lens.

Before the current study, astronomers had used the gravitational microlensing technique to discover a dozen or so of the nearly 550 known alien planets. (NASA’s Kepler mission has detected 1,235 candidate planets by a different method, but they still need to be confirmed by follow-up observations.) Sumi and his team looked at two years’ worth of data from a telescope in New Zealand, which was monitoring 50 million Milky Way stars for microlensing events. They identified 474 such events, including 10 that lasted less than two days.

The short duration of these 10 events indicated that the foreground object in each case was not a star but a planet roughly the mass of Jupiter. And the signals from their parent stars were nowhere to be found.

The discovery of 10 short-duration events in two years suggests a huge population of these unbound or distantly orbiting Jupiter-mass exoplanets throughout the galaxy, researchers said.

Sumi and his team calculated, in fact, that these planets are probably almost twice as common in our own Milky Way as main-sequence stars. And they likely outnumber “normal” planets with known host stars by more than 50 percent.

Sumi and his colleagues reported their results in the May 19 issue of the journal Nature.