Two Eyes to Hunt Stray Planet Masses


How can we measure the masses of free-floating planets wandering around our galaxy? A new study identifies one approach that combines the power of two upcoming missions.

Finding Invisible Planets

Most exoplanets we’ve found so far have relied on measurements of their host stars, either via dips in the host star’s light as the planet passes in front (transit detections), or via wiggling of lines in the host star’s spectra caused by the planet’s gravitational tug (radial velocity detections). But free-floating planets have no hosts and are therefore effectively invisible, since they don’t give off much light of their own. To find these rogues, we rely on another method: gravitational microlensing.

microlensing diagram

A diagram of how planets are detected via gravitational microlensing. In this case, the planet is in orbit around a foreground lens star, but this same diagram can also apply to a free-floating planet acting alone as the lens. Click to enlarge. [NASA]

In microlensing, the mass of a passing foreground planet — either free-floating or bound to a host star — can act as a lens, briefly gravitationally focusing the light of a background star behind it. As a result, the background star temporarily brightens (on timescales of perhaps seconds to years) in our observations. Though we never directly see the foreground planet, we can infer its presence from the spike in the background star’s brightness.

Masses from Parallax

By itself, a microlensing observation usually can’t tell us about the mass of a free-floating planet; this is because the timescale of a brightening event depends on both the mass of the lens and on the relative proper motion between the background source and the foreground lensing planet.

But if we could simultaneously observe a microlensing event from two different locations, separated by a large enough distance? Then the parallax would allow us to break that degeneracy: the differences in peak brightness and its timing at the two locations would allow us to calculate both the speed of lens relative to the source and the planet mass.

Vantage Points in Space

Where do we find two sensitive eyes located far enough apart to make this work? In space, of course!


Illustration of NASA’s WFIRST telescope. [NASA]

NASA’s Wide Field Infrared Survey Telescope (WFIRST) is set for launch in the mid-2020s, and one of its primary mission objectives is to perform wide-field imaging that may allow for the detection of hundreds of free-floating planets — and many additional bound planets — via microlensing.

As for the second eye, scientists Etienne Bachelet (Las Cumbres Observatory) and Matthew Penny (The Ohio State University) propose that ESA’s upcoming Euclid mission is exactly what we need. Euclid, launching in 2022, will have similar wide-field imaging capabilities to WFIRST, and it will be able to make complementary microlensing parallax measurements as long as the two satellites are 100,000 km or more apart.

Making Use of Gaps


Artist’s illustration of ESA’s Euclid mission. [ESA/C. Carreau]

Though Euclid’s primary science goal is to study dark energy and dark matter, Bachelet and Penny demonstrate that a modest investment of Euclid observing time — approximately 60 days during its primary mission, and another 60 days during its extended mission — during scheduling gaps would be enough to obtain the masses for 20 free-floating planets and many more bound planets.

So what are we waiting for? Let’s go learn more about the rogue planets sneaking through our galaxy!


“WFIRST and EUCLID: Enabling the Microlensing Parallax Measurement from Space,” Etienne Bachelet and Matthew Penny 2019 ApJL 880 L32. doi:10.3847/2041-8213/ab2da5