Young Exoplanet Offers Clues About Milky Way Origins

Studying the atmosphere of an alien exoplanet, astronomers are getting new clues about the origins of our own solar system.

The alien world in question is a giant extrasolar planet -- a planet outside the Solar System -- with a glowing atmosphere; located around the distant star HR 8799, which lies about 130 light-years from Earth. The HR 8799 system is home to four giant planets orbiting a relatively young, 30-million-year-old star, with each planet far larger than any world found in Earth's solar system.

Astronomers have found that the planets orbiting HR 8799 weigh in at between five to 10 times the mass of Jupiter and are still glowing with the heat of their formation, allowing researchers to directly image them.

"It's the only system in which multiple planets can individually be seen," said study co-author Bruce Macintosh, an astronomer at Lawrence Livermore National Laboratory in California.

The star system in much resembles a scaled-up version of our Milky Way, which in itself suggests that smaller Earth-size planets may be found closer in, although scientists have not yet observed any.

Leading the study was Quinn Konopacky, an astronomer at the University of Toronto. According to him, the exoplanet “even has something that kind of looks like maybe an asteroid belt interior to the closest giant planet like we have in our solar system, and something that maybe you can refer to as an Oort cloud analog out beyond the most distant gas giant — that is, a cloud of icy comets.”

According to astronomers the birth of such a massive planet at such a great distance from its parent star conflicts with the two popular models of planetary formation. In the multistep process known as core accretion, gas slowly accumulates onto a planetary core, while the mechanism known as gravitational instability involves the simultaneous creation of a planet's interior and atmosphere.

"In the traditional core accretion model of planet formation, it is difficult to form planets as large as the HR 8799 planets at such large distances from their parent star," Konopacky told SPACE.com. "Typically, in this model, objects the size of Jupiter or larger must form much closer to their parent star. This is for several reasons, but has a lot to do with there being less material at large distances from the star that can form planets."

"In the gravitational instability method of formation, it is possible to form big planets at large distances, usually because they invoke a much more massive disc of material," Konopacky added. "But the model generally predicts that there should be many more massive objects orbiting lots of other stars at these distances, and these kinds of objects have not been discovered in surveys [of many stars for exoplanets]."