Exoplanets are generally compared to Earth if they are rocky, or to Jupiter if they are gaseous. Now, for the first time, a newly-discovered exoplanet appears to be both gaseous and icy, reminiscent of Uranus, though the new planet is four times as massive. The research was led by Radek Poleski of Ohio State University, and the paper was published in The Astrophysical Journal.
This planet, located about 25,000 light-years away near the constellation Sagittarius, orbits one of the stars in a binary system. Both of the stars are less massive than our Sun at 0.7 and 0.2 solar masses, respectively. The planet is about 18 AU away from the star, very similar to Uranus's average distance of 19 AU from our Sun. Because of this distance and the size of the star, the exoplanet is assumed to be icy, just like Uranus.
Spotting exoplanets can be tricky because the glare from the star generally washes out any light reflecting from an exoplanet. The easiest way to spot planets is to identify when they transit the star, blocking out light. This technique works best with planets closest to the star, not so much with those in the outer solar system like this "exo-Uranus," which helps give context to this discovery's importance.
Instead of relying on the planet transiting the star, astronomers identified it using a technique called gravitational microlensing. This occurs when the light from a distant star is bent and focused due to the gravity of a star between the light source and the observer. Light from a planet orbiting the lensing star can also become focused under rare circumstances, which is what happened with this particular Uranus analog. Certain conditions were also in place to verify that the planet is actually part of a system.
"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," Poleski said in a press release. "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."
Uranus and Neptune, our other icy giant planet, both appear blue because of the large amount of methane ice that exist on those planets. While gravitational lensing may have revealed the exoplanet with a Uranus-like orbit, it does not provide any information about the planet's actual composition. Still, it could provide valuable information about the formation and location of our icy giants, which astronomers have struggled to explain.
"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," principal investigator Andrew Gould said. "One idea is that they did form much closer, but were jostled around by Jupiter and Saturn and knocked farther out. Maybe the existence of this Uranus-like planet is connected to interference from the second star. Maybe you need some kind of jostling to make planets like Uranus and Neptune."
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