NASA image of rogue planet
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Credit: NASA

Could trillions of rogue, starless planets be traversing our galaxy?

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Sep 2, 2020, 10:14 PM EDT (Updated)

The universe is a cosmic mess. Next to everything that crashes, explodes and keeps morphing out there, so many planets are thought to have gone rogue and scattered all over the solar system that NASA is going to launch a mission to find them.

Planets don’t just materialize in the vacuum of space. Their formation involves aggregates of dust and other materials continuously head-butting one another until they start to stick together into what could be called a planet. Asteroids, meteors and dwarf planets are often thought to be the leftovers of planetary formation that never got huge enough to qualify as planets (sorry Pluto). However, there are many actual planets which either got kicked out of their star systems or were never part of one to begin with. Now a new study suggests that there could be trillions of them floating around.

Rogue planets the size of Earth or Mars could possibly be born from the discs of gas and dust surrounding stars, but the problem is that they don’t play well with others. Gravitational interactions with other planets in their star system eventually get them kicked out of that system and send them soaring into the void. Gargantuan planets that go off on their own—think gas giants like Jupiter—might start their lives in star-forming clouds and form similarly to stars. They just never end up orbiting a star.

“If we find a lot of Mars-like rogue planets, those are going to probably be planets ejected from the systems they were born in,” Samson Johnson, a grad student at Ohio State University who led a study recently published in The Astronomical Journal, told SYFY WIRE. “If we find Jupiter-like ones, they probably formed similar to stars.”

With NASA’s Nancy Grace Roman Space Telescope, Johnson and his team will scour a stretch of the solar system between the Sun and the galactic center, about a million miles from Earth, for planets that have been forever roaming space in isolation. It will use the phenomenon of gravitational microlensing to its advantage. Gravitational lensing happens when an object warps spacetime around a star in its background, bending and magnifying the light of that star and inadvertently giving itself away to the eye of any telescope that may be staring in its direction. Roman’s hypersensitive instruments will be on the lookout for the micro-version of gravitational lensing because some of these planets could be too small to detect otherwise, especially since smaller planets are more likely to be found.

"It’s not that you can’t form rogue planets similar to gas giants like Jupiter and Saturn this way, but we expect them to be rarer that lower-mass planets, and planets significantly more massive than Jupiter and Saturn are even rarer." said Scott Gaudi, who co-authored the study. "That’s because we know that Jupiter and Saturn-mass planets are intrinsically rarer in systems where they orbit host stars, and also becuase it generally takes something more massive to kick something out of a planetary system."

Are these planets destined to be lone travelers through the aeons? Probably. Outcasts that either get themselves flung out of star systems or form alone and stay alone are unlikely to get recaptured.

"There are two other ways planets can be turned loose from their hosts," Gaudi explained. "One way is for a passing star to “pull” a planet away from its host star. Another happens when the host star evolves after it runs out of hydrogen. It will lose mass, which will cause the outer, weakly bound planets to become unbound. These two methods of forming rogue planets are probably less common."

Whether they formed alone or were kicked out, rogues are also highly unlikely to join another star system.

“While anything is possible, it would take nothing short of a cosmic miracle to get a rogue planet in a stable orbit around a star,” Johnson said, with Gaudi adding that "A planet would have to pass very close to another star to be ‘captured’. It also has to lose energy, which is can via tidal dissipation, but the probability is still very low for old stars that are generally well-separated from other stars.  In young stellar systems, the probability is higher, but still low."

Roman is equipped to find them and give those of us back on Earth an idea of what kind of physics are involved in their formation and evolution. The thing is, there is a very short window of time to detect a rogue planet that passes in front of a star, because it could be gone in as little as a few hours. The longest it could be visible for may be several days. That is how long Roman will have to glimpse the spike in stellar brightness that results from the microlensing effect, and to measure shifts in magnification that can indicate the size of the planet passing by, before the planet vanishes into the darkness again for millions of years.

There are some things this telescope will see that Johnson and his team are especially anticipating.

“We are most interested in those rogue planets that form around stars and are later kicked out of the system,” he said. “Learning about that populations could answer things about how messy the process of planet formation is. If planet formation is a really violent process, we would expect to find a lot of small, Mars-like rogue planets.”

Speaking of Earth, how likely is it that a starless planet, or an object  like 'Oumuamua, would tresspass on ur solar system? 

"It depends on how common they are," said Gaudi, "but they would have to be incredibly common for a rogue planet to temporarily visit our solar system (such that we would detect it during the time in which we’ve been monitoring the sky with sufficient area and cadence). The corollary to that is that free floating cometary-like objects like ‘Oumuamua must be very common."

Among its many scientific goals, Roman is expected to level up the estimated number of rogue planets, whose existence has not been well understood. Even exoplanets that belong to star systems were only discovered in the ‘90s. Only ground-based telescopes have tried to observe rogues until now. Roman will stare at that same area of sky for months on end, watching out for anything that might be identified as a rogue planet once the data is beamed back to Earth. There could be more free-floating planets in space than stars.

“My hope for Roman is it is going to teach us something completely unexpected,” said Samson. “We know very little about rogue planets and exoplanets is a field full of surprises, so I think there is a good chance the mission uncovers something really interesting.”

Meaning, there could be something even stranger than 'Oumuamua out there. Maybe a type of world previously only imagined in Star Trek is whizzing through the Milky Way right now. Gaudi believes anything is possible.

"In the field of exoplanets," he said, “Expect the unexpected.”

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