A Guide to Lonely Planets in the Galaxy
by Nadia Drake, Posted at National Geographic web site on March 12
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.