The object is a kind of "super Jupiter" – a gas giant about 10 times bigger than the biggest planet in our solar system, says Marie-Eve Naud, a PhD student at the University of Montreal and lead author of a scientific report describing the planet. The study is being published in the Astrophysical Journal this week.
GU PSc b is 2,000 times farther from its star than the Earth is from the sun, 67 times farther than Neptune and 50 times farther than Pluto — more distant than any planet ever discovered by a long shot, said René Doyon, a University of Montreal professor who is Naud's co-supervisor and co-author of the report.
But despite the vast distance between them, the planet is bound to its star via gravity, Doyon told CBCNews.ca. "The planet is actually moving with its star."
The researchers estimate that the planet completes its orbit around the star about once every 80,000 years. The star itself is located about 155 light years away, in the constellation Pisces, and is a small, young one, with just a third the mass of our sun.
May not be a planet
"Usually you don't expect big planets around small stars," Naud said.
On the other hand, the unusual object is so big that it may not be a planet at all. It may instead qualify as a brown dwarf or a "failed star" too small to ignite the nuclear reactions that power stars.
"Either way this is exciting," Doyon said. If it's a planet, it shows that planets can form farther away from stars than previously thought, and may not always form from the "planetary disk" of dust near a star. If it's a brown dwarf, it shrinks the known size limit of objects that can form in a way similar to the way stars form.
Regardless of what it is, it is physically similar to a planet and is very valuable to scientists seeking to know more about planets outside our solar system, Doyon said.
"These are truly jewels in the sky. We can study them in gory detail."
Young hot and bright
Because it is so far from its star, the planet can be easily observed without interference from the star's glare. That means it was much easier to determine things like the planet's size and temperature, said Naud.
The scientists also looked for "fingerprints" in different colours of light coming from the planet. Some of those indicated it is so young that it is still cooling and contracting to its final size, Naud said.
The researchers even detected water and methane in the planet's atmosphere, Doyon added.
The team found GU PSc b during a survey of young stars launched by Naud's primary supervisor, Étienne Artigaud. He hoped that those stars might be circled by young planets, which are warmer, brighter and easier to see than older planets.
The new planet itself has a temperature of about 700 to 800 C despite the distance from its star. But because it is big and gassy, it wouldn't be habitable even after it cools to a more comfortable temperature.
"What could be habitable is a moon around that planet," said Doyon. He added that Jupiter's moon Europa would have liquid water on its surface if it orbited Gu PSc b. However, the comfortable conditions wouldn't last long, as the planet would continue to cool rapidly as it got older.
The team found and studied the new object using the Observatoire Mont-Mégantic in Quebec, the Canada-France-Hawaii Telescope and the W.M. Keck Observatory in Hawaii and the Gemini Observatories in Chile and Hawaii.
Although the discovery is unlike any planet seen before, similar objects may be common in the universe, Doyon said. He added that researchers have already found another object much like it around another star.