Many different kinds of exoplanets have been found by astronomers, from giant “hot Jupiters” and “super Earths” to smaller rocky worlds like Earth or Mars. Now, another type has been discovered, an “ice giant” similar to Uranus or Neptune in our own Solar System. The planet is about 25,000 light-years away in the constellation Sagittarius and is one of the first found that appears to be similar to the ice giant planets in our Solar System, Uranus and Neptune, which are part gas and part ice in composition. The discovery was made by an international team of astronomers, led by Radek Poleski, a postdoctoral researcher at Ohio State University.
The planet is about four times as massive as Uranus, but orbits its star at almost exactly the same distance as Uranus is from the Sun. This leads astronomers to think it is likely similar in composition, although it is difficult to know for sure since the planet is so far away. Both Uranus and Neptune are composed primarily of hydrogen, helium, and methane ice.
The discovery may help astronomers understand how the ice giant planets formed in our own Solar System. According to Andrew Gould, professor of astronomy at Ohio State University: “Nobody knows for sure why Uranus and Neptune are located on the outskirts of our solar system, when our models suggest that they should have formed closer to the Sun. One idea is that they did form much closer, but were jostled around by Jupiter and Saturn and knocked farther out.”
Unlike Uranus and Neptune, however, this planet orbits one star in a binary star system. “Maybe the existence of this Uranus-like planet is connected to interference from the second star,” Gould added. “Maybe you need some kind of jostling to make planets like Uranus and Neptune.” The star the planet orbits is about two-thirds the mass of our Sun, and the second star is about one-sixth as massive. Exoplanets have been found in binary star systems before, but the fact that this Uranus-like planet is also in one makes it that much more interesting.
The planet was discovered using a technique called gravitational microlensing, where the light of a distant star is focused and magnified like a lens by the gravity of a closer star. Of the various techniques available for detecting exoplanets, microlensing was the only one suitable for detecting this type of planet. As noted by Poleski: “Only microlensing can detect these cold ice giants that, like Uranus and Neptune, are far away from their host stars. This discovery demonstrates that microlensing is capable of discovering planets in very wide orbits.”
The detection and confirmation of the planet came in two stages, as part of the Optical Gravitational Lensing Experiment (OGLE). The first in 2008 revealed the main star and found the planetary candidate. A subsequent observation in 2010 confirmed the planet as well as the second star in the binary system. Both of the observations were done using the 1.3-meter Warsaw Telescope at Las Campanas Observatory in Chile.
“We were lucky to see the signal from the planet, its host star, and the companion star. If the orientation had been different, we would have seen only the planet, and we probably would have called it a free-floating planet,” Poleski added.
The research team will continue to search the OGLE database, containing 13,000 microlensing events, for more exoplanets, including additional ice giants.
This discovery follows another one announced last July and reported in AmericaSpace. In that case, an ice giant candidate, Kepler-421b, was found orbiting an orange, K-type star about 1,000 light-years away in the constellation Lyra. That planet is about the same size as Uranus and has the longest known year of any exoplanet so far, orbiting its star every 704 days. These discoveries suggest that similar ice giant worlds may be fairly common in other solar systems. The fact that our own Solar System has two of them also supports this idea.
The new findings have been published in The Astrophysical Journal.