Earth-like exoplanets might be more common throughout the Milky Way than previously believed, astronomers report in a new study.
The researchers discovered an unusual super-Earth orbiting its star at a Jupiter-like distance, an orbital range for which only the frequency of larger planets – gas giants and ice giants – has been determined so far.
“We found a ‘super-Earth’ – meaning it’s bigger than our home planet but smaller than Neptune – in a place where only planets thousands or hundreds of times more massive than Earth were found before,” says lead author and astrophysicist Weicheng Zang of the Harvard and Smithsonian Center for Astrophysics (CfA).
In addition to finding this seemingly quirky world, the authors combined their discovery with a larger sample of exoplanet data from a microlensing survey. Their findings indicate this planet might not be quite so quirky after all.
The researchers studied changes in apparent brightness from the planet’s host star, which they incorporated into broader data from the Korea Microlensing Telescope Network (KMTNet) survey, a trio of telescopes located in Australia, Chile, and South Africa.
By examining mass ratios between a large volume of exoplanets and host stars, the researchers shed new light on our galaxy’s planetary demographics.
Their results suggest super-Earths are not limited to short-period orbits near their host stars, which is where they’ve primarily been found. These intriguing exoplanets can also exist farther away, with orbital periods more akin to those of our Solar System’s gas giants.
It’s generally harder to detect planets orbiting farther from their stars, but based on this study, Zang and his colleagues estimate one out of every three stars in the Milky Way should host a super-Earth with a Jupiter-like orbit.
“Scientists knew there were more small planets than big planets, but in this study, we were able to show that within this overall pattern, there are excesses and deficits,” says co-author Andrew Gould, an astronomer at Ohio State University. “It’s very interesting.”
The study relied on a phenomenon called gravitational microlensing, in which a massive celestial object (serving as the lens) passes between an observer and a bright background object like a star.
If the lens is massive enough, it gravitational field will warp spacetime enough to cause the path of light from the background source to curve on its way to the observer, like light bending through a magnifying glass. This creates a temporary spike in the object’s brightness, which may last for minutes or months, depending on the alignment.
The new study focuses on a microlensing event known as OGLE-2016-BLG-0007, first detected in early 2016.
Microlensing events are rare, and only a fraction of known exoplanets have been detected this way. The technique is well-suited for revealing exoplanets orbiting farther from their stars, however.
The new study is the largest of its kind to date, featuring three times as many exoplanets as previous samples, including many smaller ones.
While previous research has shown how stars can host a variety of exoplanet sizes in relatively tight orbits, the new study points to comparable planetary diversity – and profusion – in the outer regions of these planetary systems, too.
“This measurement of the planet population from planets somewhat larger than Earth all the way to the size of Jupiter and beyond shows us that planets, and especially super-Earths, in orbits outside the Earth’s orbit are abundant in the galaxy,” says co-author Jennifer Yee, an observational astronomer at the CfA’s Smithsonian Astrophysical Observatory.
The term ‘super-Earth’ typically refers to the mass of an exoplanet, not its surface conditions or habitability, about which few details are available.
Still, research like this may help demystify planetary formation and distribution in the Milky Way, building upon what our own Solar System can teach us.
“This result suggests that in Jupiter-like orbits, most planetary systems may not mirror our Solar System,” says co-author Youn Kil Jung of the Korea Astronomy and Space Science Institute that operates the KMTNet.
These findings suggest our galaxy may teem with a wide variety of exoplanets. It also offers clues about how the different types of exoplanets form, but we still need a lot more data – which is easier said than done.
“Finding a microlensing star event is hard. Finding a microlensing star with a planet is hard squared,” says co-author Richard Pogge, an astronomer at Ohio State. “We have to look at hundreds of millions of stars to find even a hundred of these things.”
The study was published in Science.
