In 2006, the International Astronomical Union — the professional organization for astronomers — put itself through a painful and, for many members, embarrassing exercise: It tried to come up with a scientific definition for "planet," a word that dated from ancient times.
The crisis arose as astronomers found more and more worlds, some larger than Pluto, orbiting the Sun in the frozen wastes of the outer solar system. Should they be called planets? Would the solar system become a very crowded place? Did Pluto still qualify? What about all those people who grew up believing there were nine planets? (“My Very Educated Mother Just Served Us Nine Pizzas.”)
After heated argument, they came up with this: "A 'planet' is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit."
Kaboom. Things changed with a paper in this week's edition of the journal Nature.
Study author Takahiro Sumi, an astrophysicist at Osaka University in Japan, reports that he's found at least ten Jupiter-sized objects wandering the nearby galaxy, apparently not part of any solar system. "These planetary-mass objects have no host stars that can be detected within about ten astronomical units," he writes. He only found them by watching to see if anything moved in front of distant stars, distorting their light.
If they were orbiting stars, they would most likely fit the IAU's definition of "planet." They're probably large enough to be round, but not large enough to generate heat and light as stars naturally do. They have enough gravity that they probably swallow up any smaller space junk wandering their way.
They may have formed when stars did, but were, for some reason, flung free of them. And here's the zinger: Sumi and others say that based on his two-year survey, such worlds may be more common than stars in our galaxy.
No saying what these bodies are really like close-up, but scientists suggest they must be dark, frozen, and desperately lonely in the eternal night. They may have moons. They may have seas of liquid that would be gas if they had suns to warm them, or sheets of ice made from compounds that would be liquid if there were ever daylight. The view of the stars around them must be staggering.
Someone –perhaps Sumi — will have to come up with a name for them.
(Image: Artist's conception of a planet without a host star. Credit: NASA/JPL-Caltech.