Although astronomers have found and confirmed hundreds of planets in the Milky Way galaxy, discovering one resembling our "ice giant" planets Uranus and Neptune is a rare occurrence: so rare, in fact, that we haven't found one before.
Now, though, for the first time, astronomers have spotted such an ice giant planet in our galaxy that is about 25,000 light-years from Earth.
Planets like Uranus and Neptune contain mostly hydrogen and helium, but also large amounts of methane ice, which makes them appear blue. Of course, this new planet is so far away that we can't see its color, but astronomers determined that it's an "ice giant" based on its orbit around its sun, which is nearly identical to that of Uranus.
This new planet may also explain one of the mysteries about Uranus and Neptune. Because it sits in a binary star system, its orbit is often disturbed by the second star. This disturbance, or "jostling," could be responsible for its formation.
"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," says Andrew Gould, professor of astronomy at Ohio State. "One idea is that they did form much closer, but were jostled around by Jupiter and Saturn and knocked farther out."
Although the new planet is four times the mass of Uranus, it orbits its star at about the same distance that Uranus orbits our solar system's sun.
So how was this planet found? A phenomenon called gravitational microlensing played a large part in the discovery. This is when the gravity of a star bends light around another object, magnifying it so that it's easier to see from Earth. The Warsaw Telescope in Chile observed two such events, first finding the main star, and then the second and confirming the existence of this new planet.
After analyzing this data, astronomers calculated the stars' and the planet's masses, as well as their distances in relationship to each other. Only the microlensing effect allows us to gather so much detail about these faraway worlds.
"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," says Poleski. "We were lucky to see the signal from the planet, its host star, and the companion star."