ROGUE PLANETS

ORPHAN PLANETS RIPPED AWAY FROM THEIR STELLAR PARENTS OR BORN IN ISOLATION MAY VASTLY OUTNUMBER STARS

Ever since astronomers first began discovering planets outside of the Solar System, dubbed ‘exoplanets’, in the mid-1990s, our picture of the Solar System as a ‘typical’ arrangement of planets and stars has been turned on its head. Among the 5,600 or so exoplanets that we have discovered over the course of the last three decades are scorching hot worlds bombarded by so much stellar radiation that they rain molten metal, planets that race around their stars so fast they can fit several years into a single day and planets squashed by the gravity of their stars so much they have flattened discs. While all of these discoveries have changed our perspective of the Solar System and its planets being ‘typical’ and may have even made our corner of the Milky Way look slightly mundane by comparison, they all share a similar theme: planets orbiting stars – albeit sometimes strangely, violently or chaotically.

In 2012, the universe let humanity know that even this ever-so-familiar concept can’t be taken for granted. That’s the year that astronomers operating the Canada-France Brown Dwarfs Survey discovered the first possible ‘rogue planet’, a cosmic orphan that drifts through the cosmos away from a parent star. Rogue planets go by a range of alternative names, including interstellar, nomad, orphan, starless, unbound or wandering planets, but the more official names for these worlds are free-floating planets (FFPs) or isolated planetary-mass objects (iPMOs).

The classification of iMPOs also includes so-called ‘failed stars’ called brown dwarfs. These are objects with masses up to around 12 times that of Jupiter – just outside the range of the largest planets – which form in isolation like stars but never quite gather enough mass to trigger the nuclear fusion of hydrogen to helium in their cores. Because they also wander the universe alone, it can be difficult to distinguish brown dwarfs from high-mass rogue planets.

The few hundred planetary rogues discovered thus far are merely the tip of the iceberg. Last year, a nine-year survey called Microlensing Observations in Astrophysics (MOA), conducted by scientists from NASA and Japan’s Osaka University, suggested that rogue planets in the Milky Way far outnumber planets that orbit stars, also outnumbering those stars by around 20 times. “Rogue planets could vastly outnumber the stars in the Milky Way. There are 1 to 300 billion stars in the Milky Way, so there could be trillions of free-floating planets,” NASA’s Jet Propulsion Laboratory postdoctoral fellow Samson A. Johnson tells All About Space. “The fact that we’re finding any at all means there’s a lot, but just exactly how many is a lot is hard to say right now.”

“Because they also wander the universe alone, it can be difficult to distinguish brown dwarfs from high-mass rogue planets”

Just how do these planets come to be cosmic orphans? There are two possible ways that planets can end up wandering the Milky Way isolated from a parent star. Which mechanism led to the rogue planet’s existence as a cosmic orphan depends strongly on its mass. Johnson explains that large rogue planets with masses around that of Jupiter and greater are likely to form in a similar way to stars.

Giant clouds of gas and dust called interstellar molecular clouds exist within galaxies and between stars. When patches of these clouds cool and become overdense, they collapse to form protostars. If these stars can gather enough mass from what remains of the cloud, they become full-fledged stars; if not they become brown dwarfs. But the collapse of these dense patches of molecular clouds can also create isolated Jupiter-mass rogue planets that never had a parent star.

For rogue planets that have masses closer to that of Earth, the story is different. “The other way that you can form free-floating planets is by ejecting them from planetary systems,” Johnson says. “There are two ways this could happen. First, it could happen when the planetary system is forming.” Planetary formation around an infant star is a very chaotic process with lots of different dynamics going on and objects ranging in size from dust grains to planets just flying around. “There’s a lot of things that can happen,” Johnson continues. “The most likely thing is that some massive planet, the bully of the system, will start kicking things around just because it has a lot of mass to throw about. This planetary bully can put lower mass objects on larger orbits, or it can toss them out of the planetary system altogether.”

These ejected objects are likely to have masses around that of Mars, with some even possessing higher masses equivalent to Earth’s. As for the speed at which the planets are ejected, Ohio State University physicist Scott Gaudi explains that it depends on where around its star the planet was located. “If they were ejected from the outer part of the planetary system, then rogue planets will have a speed that’s only slightly different than the velocity of the nearby stars – a few tens of kilometres per second,” Gaudi explains. “To get ejected from closer in, rogue planets have to be kicked out with a much higher velocity.” As an example, for Earth to be ejected from the Solar System its velocity around the Sun of about 30 kilometres (18.6 miles) per second would have to be squared.

All this means that discovering rogue planets and then tracing them back to their former planetary systems has the potential to teach scientists a great deal about planetary formation. “I think rogue planets are interesting from a planet formation perspective because the low-mass ones are most likely to be orphaned by ejecting them from their planetary systems during formation,” says Gaudi. “So rogue planets provide a way of measuring how chaotic and violent planet formation might be.” Gaudi also adds that by accounting for the total mass of free-floating planets, scientists could learn something about how much mass is contained in the discs of gas and dust that surround infant planets in so-called ‘protoplanetary discs’, how much of it remains in mature planetary systems and how much is ejected.

Johnson explains that the other way a planet can be tossed from a planetary system happens at the opposite end of a star’s life cycle, during its red giant phase. After billions of years of burning hydrogen at their cores, stars eventually run out of this fuel for nuclear fusion. As these stars undergo these death throes, their cores collapse under the influence of their own gravity, but the outer layers where nuclear fusion is still happening puff out. This is known as the red giant phase of the star’s life, and it can see the star swelling to between 10 and 100 times its original width. It’s followed by the further dispersal of this shed stellar material, leaving the stellar core as a white dwarf stellar remnant. This evolution and the upheaval it causes in the planetary system can also lead to the ejection of lowmass planets.

Exoplanets are usually spotted using their effects on their parent stars. For example, an exoplanet that crosses the face of its star will cause a tiny and brief drop in that star’s light output. Alternatively, the gravitational tug of a planet on its star can create a tiny wobble in the star that can also be seen in its light output. The fact that rogue planets have been orphaned from their stellar parents or never had them to begin with means that they can’t be detected by the effects they have on a close star. This has made these free-floating worlds highly tricky to pin down.

Larger rogues around the size of Jupiter have been imaged directly thanks to the small amount of infrared light they emit, but smaller planets that freeze as a result of their lack of heat from a star and their lack of internal heat can’t be seen in this way. Fortunately, when these orphan planets pass background stars, they can trigger an effect that emerges from Albert Einstein’s theory of general relativity called gravitational microlensing. “I work on gravitational microlensing, which is a technique to find exoplanets. It has the advantage over other methods that it doesn’t require detecting light from the planet or even the host star,” Gaudi says. “In principle, it could detect free-floating planets with quite low masses. It’s really the only way of detecting very low-mass planets that are not bound to their host star.”

General relativity says that objects with mass cause a warping of the very fabric of space. As light passes this warp, it’s curved. That means when a massive object comes between Earth and a background object and acts as a gravitational lens, the path of light from the object is curved. This has the effect of making the background object adopt a new apparent position in the sky. As the closer light passes to a gravitational lens, the more it’s curved, an intervening object can cause the arrival time of light from the same object to vary, which can cause a background object to appear in multiple places in the same image. The catch is that the greater the mass of an intervening object, the more extreme the curvature of space it generates. That means while entire galaxies acting as a gravitational lens cause a huge amount of curvature and a large shift in apparent position, the effect of a planet with a mass around that of Earth is insignificantly smaller and can’t be seen by looking at a single background star. It can be calculated, however, by looking at millions of stars and averaging the effect across them.

“Rogue planets provide a way of measuring how chaotic and violent planet formation might be”

Scott Gaudi

Both Johnson and Gaudi are heavily involved with the Nancy Grace Roman Space Telescope, which promises to have a massive impact on our knowledge of rogue planets when it launches in 2027. The space telescope will do this by staring at millions of stars in the Milky Way to conduct a vast microlensing survey sensitive to relatively low-mass free-floating planets. “This is the next large astrophysics mission to be launched by NASA, and it will conduct a microlensing survey to search for planets,” Gaudi says. “We estimate it will find around 1,500 bound exoplanets, but it’ll also find hundreds of free-floating planets with masses all the way down to the mass of Mars – maybe even lower if rogue planets of lower mass are quite common.”

Scientists are currently predicting that Roman will be able to spot a staggering 400 Earth-mass rogue planets in the Milky Way, massively boosting our understanding of these cosmic orphans. “I think there’s still a lot to learn about rogue planets,” Johnson concludes. “Being able to tie down the occurrence rate of these objects is going to lead to some big surprises.”