Everybody knows what a planet is, right? It's a big ball of rock or gas—or something—in obit around a star, no? Well, maybe, maybe not, especially when it comes to exoplanets circling distant stars.
There's an official definition, of course, which was issued by the International Astronomical Union in 2006 and which saw Pluto kicked to the solar system curb, demoted from planet status to dwarf planet.
Problem is, the IAU definition only applies to bodies within our solar system, which leaves the thousands of distant exoplanets in "definitional limbo," says planetary astronomer Jean-Luc Margot at UCLA.
A new definition is needed, he says, and he's proposed one in a research paper to appear in the Astronomical Journal, the contents of which he presented at a meeting of the American Astronomical Society.
The IAU definition has as its litmus test the simple question: can the object in question clear its orbit? In other words, can it accumulate or dominate the smaller bodies that lie in its orbital path?
All the planets in our solar system pass that test except Pluto, which is why it's now officially a dwarf planet.
However, the test runs into problems when it comes to exoplanets, which are far away and difficult to detect—and almost impossible to ascertain if they're "clearing their orbits."
"One should not need a teleportation device to decide whether a newly discovered object is a planet," he says.
Margot proposes a simpler test, a formula that requires only knowledge of the candidate object's orbital period, its mass and the mass of the host star—information easily confirmed using telescopes on Earth or in space.
That information would be sufficient to determine whether an object is capable of clearing the immediate regions of its orbit within a specific time frame, such as the lifetime of its parent star, he says.
The formula would confirm all eight of our solar system's planets as well as 99 percent of the approximately 5,000 exoplanets found to date.
That flood of exoplanet discoveries should push the IAU toward refining and extending the official accepted definition of a planet, Margot says.
People who were disappointed with Pluto's demotion won't get much help from Margot; the dwarf planet comes up short in his formula as well as under the IAU definition.
So do Pluto's fellow dwarf planets Ceres and Eris, and there's no room for debate, he says.
"The disparity between planets and nonplanets is striking," he explains. "The sharp distinction suggests that there is a fundamental difference in how these bodies formed, and the mere act of classifying them reveals something profound about nature."
