Study suggests galaxy has fewer rogue gas giant planets than scientists thought

As many as 75 percent of rogue planets may be Earth-sized rocky and icy worlds.
By Laurel Kornfeld | Jul 29, 2017
A new study using data collected by the Optical Gravitational Lensing Experiment (OGLE) suggests the Milky Way galaxy contains fewer rogue gas giant planets free floating in space than previously thought.

Unlike stars, planets do not emit their own light, so finding those that do not orbit any star is challenging for astronomers.

In a 2011 study, an international team of scientists conducted a study in which they concluded the galaxy has twice as many rogue gas giants around the size of Jupiter than main-sequence stars.

The current research team, which includes scientists from Warsaw University Observatory, Ohio State University, and the University of Warwick, used OGLE data taken between 2010 and 2015 that analyzed the light curves of more than 50 million stars to reach a different conclusion.

Although rogue planets cannot be directly observed, OGLE identified them via the technique of gravitational lensing. When an object moves in front of a more distant star or galaxy, light from the distant object is bent by the gravity of the forefront object, creating a lensing halo that instruments on Earth can see.

By measuring the duration of the lensing effect, scientists can calculate the size of the forefront object, in this case, the rogue planet.

In contrast to the 2011 study, which analyzed 474 high-quality microlensing events, the current one observed 2,617 such events.

Significantly, they found rogue Jupiter-size planets to be far rarer than claimed by the earlier research team.

Of the larger free-floating planets, about 25 percent detected were likely to be gas giants.

These findings suggest the galaxy may host a much larger number of smaller, Earth-sized rocky and icy planets. Smaller planets have weaker gravity than large ones and are more easily ejected from their original star systems than gas giants.

Gravitational microlensing is currently the only method capable of detecting rogue planets with approximately the mass of Mars.

The research team published its findings in the journal Nature.

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