Gravitational Lensing Reveals "Iceball Earth" 13,000 Light-Years Away

Astronomers have discovered an "iceball Earth" orbiting a star 13,000 light-years away. Multiple eyes on the sky have revealed some of this world's secrets.

NASA’s Kepler spacecraft steals most of the exoplanet thunder these days for its prolific catalogue of planetary discoveries. But beyond Kepler’s reach are planets such as OGLE-2016-BLG-1195Lb, an “iceball Earth” more than 13,000 light-years away, the same distance from its star as we are from our Sun.

OGLE stands for Optical Gravitational Lensing Experiment, a ground-based telescope in the Atacama desert run by the University of Warsaw, Poland, that’s dedicated to finding and following microlensing events, a gravitational lensing phenomenon that occurs on small, even planet-size scales.

Microlensing occurs when one star passes in front of another more distant star seen along the same line of sight. The mass of the closer star lenses the light from the more distant star, which appears to brighten over the course of hours to days to weeks. Since microlensing is transient and relies on precise alignment, the best way to catch an event is to survey a huge, area on the sky, densely packed with stars, night after night. OGLE monitors the brightness of hundred of millions of stars in the Milky Way’s central bulge and the Magellanic Clouds.

While OGLE’s primary mission is to probe the nature of dark matter, the microlensing events it finds can also reveal the presence of planets. If the intervening, lensing star has a planet, it creates a shorter, secondary brightening of the distant star. In fact, while Kepler was best suited to finding planets closely orbiting their stars, OGLE is better suited to finding planets farther away from their stars, thanks to the geometry required for lensing. The newest discovery is the planet OGLE-2016-BLG-1195Lb.

In late June 2016, OGLE detected the telltale brightening of a microlensing event and sent out an alert requesting several other facilities to observe it simultaneously. Both the ground-based Korea Microlensing Telescope Network (KMTNet) and NASA’s space-based Spitzer Telescope jumped on board to monitor the event over the course of a month. At the time, astronomers didn’t know about the presence of a planet. It was only when analyzing the data after the fact that the team saw its signature.

For its part, OGLE alone can only measure the ratio of the lensing star’s mass to the mass of its planet. It was the additional data from KMTNet and Spitzer, which created a sort-of stereoscopic vision (think classic Viewmaster), that enabled astronomers to calculate the mass of individual objects in the system.

It turns out, the newly discovered planet is not only roughly the same mass as Earth, it’s also in roughly the same orbit around its star. But while we lie in our Sun’s habitable zone, OGLE-2016-BLG-1195Lb lies so far beyond the habitable zone of its smaller star, it’s likely a permanent ball of ice. If you’ve ever wished for winter all year-round, this is the planet for you.

OGLE-2016-BLG-1195L, the star itself, is only 7.8% the mass of our Sun, or just under 100 times the mass of Jupiter, which puts it in the grey area between red dwarf stars and brown dwarfs (which are too small to fuse hydrogen). That means that not only is it always winter on this iceball Earth, it’s a dimly lit one at that.

Given that red dwarfs are the most common type of star in the galaxy, the more we can learn about their likelihood to host planets, the better we can try and understand planet formation. We still don’t have a very good idea of how a star’s environment (presence of binary companion, density of nearby stars, intense radiation from stellar neighbors) might affect its ability to host a planetary system.

Projects such as OGLE, which focuses its observations on denser environments like the galactic bulge, could help address the question of whether or not the frequency of planets in the disk of the Milky Way differs from the frequency in the bulge. A recent study of 20 OGLE planets suggests that planets might be less abundant in the bulge, but the question is far from resolved. Stereoscopic observations of each new microlensing event with Spitzer and even Kepler (in its K2 phase) could shed a lot more light on this issue.