Rogue planets are homeless worlds. They have neither sunrises nor sunsets, because unlike the planets we’re more familiar with, these lonely worlds aren’t tethered to a star. Instead, they travel in solitary arcs around the Milky Way’s core.
Earlier this week, Cosmos: A Spacetime Odyssey, introduced many of its viewers to the concept of these lonely planets.
“The galaxy has billions of them, adrift in perpetual night. They’re orphans, cast away from their mother stars during the chaotic birth of their native solar systems,” Neil DeGrasse Tyson says, as a planet emerges from the darkness. “Rogue planets are molten at the core, but frozen at the surface. There may be oceans of liquid water in the zone between those extremes. Who knows what might be swimming there?”
In the days that followed the show’s premiere, social media sites lit up with questions from viewers wondering what, exactly, these rogue worlds are — and could there really be billions of them, as Tyson said?
(The answer is yes. Probably.)
For decades, astronomers hypothesized that free-floating planets existed. But scientists needed a way to find them. The two most well-known ways of finding exoplanets rely on telltale signals coming from the planets’ stars – either wobbles caused by the gentle tugs of an orbiting planet’s gravity, or the slight dimming produced when a planet passes between Earth and its star.
So how do you find planets that have no stars?
For now, the best methods include looking for a young rogue’s heat in the infrared, and a technique called gravitational microlensing that works well for older, cooler planets, says astronomer David Bennett of the University of Notre Dame. Microlensing takes advantage of gravity’s ability to bend and mess with light. If a massive object – say, a rogue planet – passes between a star and Earth, the planet can act as a lens, curving and tweaking the star’s light as seen from Earth. In general, the more massive the planet, the more affected the light.
So far, using either method, we can’t easily detect starless planets that are smaller than a Jupiter, or at least 300 times the mass of Earth.
Anyway, early observational hints of these untethered worlds turned up in the late 1990s, when a team of Japanese astronomers found evidence for warm, planetary mass objects in the Chamaeleon cluster, about 500 light-years away. Other teams soon reported more rogue candidates, in a cluster near the star sigma-Orionis, in the Orion nebula, in the Taurus star-forming region. More recently, in 2012, astronomers described a hot (700 degrees Celsius) homeless planet, clunkily named CFBDSIR2149-0403, just 100 light-years away.
Evidence for the “billions” parts of Tyson’s statement arrived in 2011. A microlensing study published in Nature suggested the Milky Way contains at least 400 billion star-less worlds, that the lonely planets are more common than stars like our sun. Data from two microlensing consortiums, known by the acronyms OGLE and MOA, pointed toward 10 possible free-floating planets, spotted over a two-year-long survey aimed toward the Milky Way’s galactic bulge.
Based on comparisons between the surveys’ detection efficiency, the probability of microlensing events, and the expected amount of lensing caused by planets and stars, the team concluded that these planetary lenses were everywhere. “There are statistical uncertainties in the analysis,” says Bennett, a member of the MOA consortium. “Four hundred billion planets is probably a good lower limit.”
But not everyone is convinced. Despite careful work by the authors, it’s still possible the objects detected are just very far from their stars, that they’re brown dwarfs (a type of low-mass pseudo-star that failed to ignite nuclear burning in its core), or that the galactic population estimates are off.
Since 2011, though, MOA has been hard at work analyzing larger data sets and refining estimates for how many free-floating planets populate the Milky Way. So far, Bennett says, new estimates appear to support the original finding that these rogue planets are really common. And, there are hints that we may soon be able to find smaller, untethered planets about the mass of Neptune – much too small to be mistaken for a failed star.
Now, about those chaotic early years. Astronomers suspect that many free-floating planets are wandering through interstellar space because they’ve been kicked out of their home stellar systems. This process tends to happen early in a system’s history, says astrophysicist Greg Laughlin of the University of California, Santa Cruz. As planets in young systems settle into their orbits, their gravitational jostling can end up sending a sibling or two into space.
It’s possible that something like this happened in our solar system. Theories describing the early solar system don’t really work unless a fifth giant planet – another Uranus or Neptune – were present at the start (one of the problems with these models is that Earth sometimes ends up running into Venus, which we know didn’t happen). Later, as the planets begin to migrate, that fifth giant is kicked out of the solar system and sent flying into space.
Where it is now is anyone’s guess. “The damn thing could be half-way across the galaxy, for all we know,” says Konstantin Batygin, a post-doc at the Harvard-Smithsonian Center for Astrophysics.
Ok. What about the second part of Tyson’s quote? Could these worlds really have molten cores and subsurface oceans?
Perhaps surprisingly, the answer is yes. This part of the narrative echoes a paper published in 1999 by Caltech planetary scientist David Stevenson, who considered how Earth-mass planets cast from their solar systems might fare in outer space. Stevenson suggests that if these planets retained a hydrogen atmosphere, they could stay warm enough to have liquid water on their surface. A subsurface ocean could be present even without an atmosphere. And, larger planets are generally warmer than smaller planets, says Stevenson, who calculated that a cast-off Jupiter would only cool by about 15 Kelvin at its surface.
Let’s hope there are critters — preferably plesiosaurs or laser sharks — swimming in those rogue, subsurface seas.