Characteristics of planets vary greatly, and defining one isn't always easy (just ask Pluto). But one trait astronomers usually agree on is that planets orbit a parent star, as Earth does the sun. But then came PSO J318.5-22, the scientific name for an unusual object recently discovered by an international team of astronomers, including Bucknell professor Katelyn Allers.
"Everything we've measured for this object is planet-like — its mass, its temperature, its brightness," Allers explained. But there's something missing. "It doesn't have a parent star. We call it the lonely planet because it's just traveling through the galaxy, basically by itself. It really is going to force us to change the way we think about the definition of a planet."
The discovery was made using the Pan-STARRS 1 (PS1) wide-field survey telescope on Haleakala, Maui. Follow-up observations using several other telescopes showed that the object has properties similar to those of gas-giant planets found orbiting around young stars, only PSO J318.5-22 doesn't have a host star. When the team realized what what it had potentially found, it relied on Allers' expertise in determining temperatures and compositions to help confirm it is indeed a planet.
"We have never before seen an object free-floating in space that that looks like this," said team leader Michael Liu of the Institute for Astronomy at the University of Hawaii at Manoa. "I had often wondered if such solitary objects exist, and now we know they do."
The absence of a parent star makes this "lonely planet" particularly valuable for research purposes; it can be studied without interference from the massive amount of light that a sun-like star gives off.
When we look at most exoplanets (planets outside of our solar system), we're also looking at an extraordinary amount of light from its nearby parent star, and that can make studying certain characteristics of the planet challenging," Allers said. "We don't have that problem with PSO J318.5-22, so we can get a much more detailed look at the characteristics of this planet."
The planet also offers researchers another useful trait — its age. By regularly monitoring the position of PSO J318.5-22 over two years, the team directly measured its distance from Earth — about 80 light years (or roughly 480 billion miles). Based on that distance and its motion through space, the team concluded that PSO J318.5-22 belongs to a collection of young stars called the Beta Pictoris Group that formed about 12 million years ago.
"The ability to determine an object's age is a rarity in astronomy. That allows us to use this planet as a benchmark for observing other objects," Allers said, adding that at about 12 million years old, the planet is relatively young — our sun, for example, is about 4.5 billion years old.
Observations of the young planet already have Allers and her colleagues rethinking about how a planet is born and evolves — how it changes as it ages. Astronomers will continue to monitor PSO J318.5-22 to see what secrets of space it reveals.
"We knew right away that this was something special," Allers said. "It's really a fascinating find."