As astronomers discover more and more planets in galaxies far, far away, they are increasingly confronted with a curious subset of orbs that are free-floating and not connected to or orbiting a particular star. Further complicating matters is that within that group, most of what they have found are gassy, Jupiter-sized (read: large), planets; few resemble rockier planets like our own Earth.
First discovered in 2003, these potential free-floating planets are elusive and difficult to detect from the existing ground-based observatories.
Soon, however, a revolutionary new telescope launching in 2025 may be able unlock the secrets of the darkness of space, where sunless worlds may even outnumber the stars. NASA's Nancy Grace Roman Space Telescope will be able to see even more rocky free-floating planets, potentially hundreds as small as Mars, according to research published this August in the Astronomical Journal. These lightless worlds can shine light on how planets formed and what happens to them after their star finally dies.
"The galaxy could be teeming with these free-floating planets, or maybe none," says Scott Gaudi, an astronomer at Ohio State University and an author on the new research. "There could be more Earth-mass planets than stars in the galaxy…Now we'll have the possibility with Roman to figure that out."
The Nancy Grace Roman Space Telescope, named after NASA's first chief astronomer who tirelessly advocated for new tools like Hubble and made several important contributions to the field of astronomy, will engage in a trio of core surveys. Roman will study dark energy, survey a special type of supernovae and discover numerous exoplanets through a technology known as gravitational microlensing.
This technique can reveal objects too dark to discover through other means, objects such as black holes or planets. When an object, like a planet, passes in front of a star, its gravity causes a very slight brightening to the stellar light. The faint magnification, predicted by the theory of general relativity, can provide insights into the passing magnifier. Unlike most other planetary discovery techniques, microlensing can find worlds cast off from their star, drifting through the darkness of space.
"Microlensing can find planets from a little past Earth to the center of the galaxy," says Samson Johnson, a graduate student at Ohio State University and first author on the new research. "It can find planets all throughout the galaxy."
The technique has its own limitations. Once a planet completes the lensing process, it continues to drift through the darkness of space, never to be seen again from Earth. But Johnson says that's not a huge problem—after all, astronomy is full of transient, one-time events. "You don't ask a supernova to explode again, you don't ask black holes to re-merge," he says.
While free-floating planets may saturate space, finding them is something of a crapshoot. The process requires three objects—Earth, the background star, and the undiscovered mystery object—line up precisely. Rather than looking at a single star and waiting for the odds to be in their favor, astronomers instead perform massive surveys watching hundreds of millions of stars at the same time for the subtle brightening caused by microlensing. These enormous surveys allow astronomers to discover as many as 2,000 to 3,000 potential microlensing events each year, only a handful of which are wandering planets, according to microlensing observer Przemek Mroz, an astronomer at CalTech who was not part of the new research.
Earth’s atmosphere creates interference than can make these small events difficult to observe. What sets Roman apart is that it will be orbiting in space, allowing it watch for even briefer microlensing events that represent smaller planets. Additionally, since most such telescope surveys are performed using optical light, the part of the spectrum that humans see with their eyes, they cannot peer through the dust in the center of the galaxy. Roman will rely on infrared light rather than optical, allowing it to peer into the heart of the galaxy, dramatically increasing its ability to discover free-floating worlds.
New Earth-sized worlds discovered by Roman can help researchers understand the messy process of planet formation. Previous solar system observations led scientists to suspect that the giant planets, especially Jupiter, used their gravity to hurl some of the planetary embryos and young planets out of the solar system, a process likely repeated in other systems. Roman can help to spot some of those lost worlds and determine roughly how many were ejected.
But planets aren't only lost during the first moments of their lives. Passing stars can wrangle away worlds that are only loosely connected to their star. A parent star can also drive away its planetary children as it evolves. In a few billion years, our own sun will swell up to a red giant, shedding enough stellar material that its gravitational hold on its planets will weaken, allowing some to wander away.
Some planets may even form without the help of a star. Recent studies suggest that a small enough pocket of gas and dust could collapse to form not a star but a gas giant.
While scientists can't verify the source of a single free-floating planet because none of the ejection processes leave their fingerprint on the world, a statistical look at the population should provide its own insights. Enter Roman, which will discover a wealth of new starless worlds. "If we find a bunch of Earth-mass planets, they almost certainly formed around a star," Gaudi says, because self-forming planets require more mass.
Roman's observations should provide insights about the free-floating worlds and how they became wanderers in space. "We're starting to run into the limit of what we can do from the ground with ground-based microlensing surveys," Gaudi says. "That's why we need to go to space and use Roman."