Astronomers have detected a rogue planet hurtling through space without orbiting a sun. This planet, believed to likely be roughly the mass of Mars, is the smallest of its kind discovered to date and could only be observed indirectly with a technique called gravitational microlensing, a technique often used to detect exoplanets.
The Optical Gravitational Lensing Experiment (OGLE) in Poland first reported the existence of sunless, free-roaming planets in 2011, but this one would be the smallest-ever discovered, emitting a measurable signal for a mere 42 minutes. The discovery was published in the journal Astrophysical Journal Letters on Thursday.
“This is the shortest microlensing event ever found, so the lowest mass planet ever found by microlensing” said Przemek Mróz, the first author of the paper and a postdoctoral scholar at Caltech. “This is really exciting because it's such a tiny piece of rock.”
Discovering an ordinary exoplanet typically requires observing the star that it orbits, inferring the presence of the planet in the form of changes in the light that the star emits over time. But these free-roaming exoplanets don’t have a nearby light source, so the astronomers used a method underpinned by Einstein’s theory of relativity: gravitational microlensing.
Planets bend the space-time fabric around them through their gravitational fields. When the Earth, an exoplanet, and a star are in a straight line, the exoplanet between Earth and the star acts like a magnifying glass, making the star appear brighter. The time these three objects stay in a line depends on how massive the exoplanet is, among other factors, so the duration of a signal can be used to estimate the planet’s mass.
Because it’s an exceedingly rare event for these three space objects to line up perfectly, and because OGLE has already observed multiple rogue planets using gravitational microlensing, these types of planets may be as common as stars throughout our galaxy, Mróz said.
OGLE began in 1992 as a project to search for dark matter using microlensing but has transitioned to searching for rogue planets in the past decade, Mróz said. Due to the COVID-19 pandemic, though, the OGLE telescope has not been making observations regularly from where it is housed in Chile. So Mróz and his colleagues decided to look through archival data for extremely brief gravitational microlensing signals.
In July, they found a 42-minute observation from 2016. Next, they compared their observation to data from a Korean telescope that happened to be observing the same part of the galaxy on that night and confirmed the signal wasn’t just a fluke in their instruments. The planet they found was between the mass of Mars and Earth and could not have been orbiting a star within 8 astronomical units—for reference, Saturn is 9.5 astronomical units from our Sun. Ruling out any star in this planet’s orbit for certain would be a challenge, Mróz said, since it would take years of observation for the planet to even make one trip around a star at that distance or greater.
Scientists believe these rogue exoplanets can come into existence through a couple of processes: they may form when clouds of gas collapse inward, or the planets may have been ejected from orbiting a star. In a few billion years, for example, our Sun will have lost mass and its gravitational force will have decreased to the point where Uranus and Neptune will be released from orbit and will drift into outer space.
Mróz said that he and his colleagues are continuing to analyze archival data and to look for even more low-mass rogue planets.
“Generally, the concept of free-floating planets is one I think is interesting—that you have a planet that is wandering through the galaxy without a star,” he said.
“It always strikes me that thanks to using Einstein's theory, we can detect such tiny pieces of rock floating in space. That's really cool.”