New research by scientists at NASA and Japan’s Osaka University suggests that rogue planets worlds that drift through space untethered to a star far outnumber planets that orbit stars. The results suggest that NASA’s Nancy Grace Rome Space Telescope, due to launch by May 2027, could find a staggering 400 dangerous Earth mass worlds.
David Bennett, senior scientist at NASA’s Goddard Space Flight Center says “We estimate that our galaxy is home to 20 times more rogue planets than stars trillions of worlds wandering on their own, the first measurement of the number of rogue planets in the galaxy that are sensitive to planets less massive than Earth.”
The team’s findings are based on a nine-year survey called MOA (Microlensing Observations in Astrophysics) conducted at the Mount John University Observatory in New Zealand. Microlensing events occur when an object, such as a star or planet, comes into near-perfect alignment with an unrelated background star from our vantage point.
Since anything with mass distorts the fabric of space-time, light from a distant star bends around a closer object as it passes close by. The closer object acts as a natural lens, creating a brief jump in the brightness of the background star’s light that gives astronomers clues about the intervening object that they can’t get otherwise.
Takahiro Sumi, a professor at Osaka University says”Microlensing is the only way we can find objects such as free-floating low-mass planets and even primordial black holes”.
The Earth mass rogue planet the team found represents the second discovery of its kind. An article describing the find will appear in the next issue of The Astronomical Journal. A second paper will be published in the same journal, which presents a demographic analysis that concludes that rogue planets are six times more abundant than worlds orbiting stars in our galaxy.
Team says we’ve gone from wondering if the worlds in our solar system are alone in space to discovering more than 5,300 planets outside our solar system. The vast majority of these newly discovered worlds are either massive extremely close to their host star.
World building can be messy, as all forming celestial bodies interact gravitationally as they settle into their orbits. Planetary light bodies are not so tightly bound to their star, so some of these interactions eventually eject such worlds into space. Thus begins a lonely existence, hidden among the shadows among the stars.
In one of the early episodes of the original Star Trek series, the crew encounters one such lonely planet in the middle of the so-called star desert. They were surprised to finally find Gothos, a starless planet, habitable. While such a world may be plausible, the team points out that the newly discovered “rogue Earth” is unlikely to share many other characteristics with Earths above a similar mass.
Roman’s hunt for hidden worlds
Microlensing events that reveal lone planets are extremely rare, so one key to finding more is to cast a wider net. That’s exactly what Roman will do when it launches in May 2027.
Naoki Koshimoto, who led the paper announcing the revelation of the potential Earth-based villain world says “Roman will be sensitive to even lower-mass rogue planets because he will be observing from space”.
Previous best estimates, based on planets found orbiting stars, suggested that Roman would spot 50 wild Earth-mass worlds. These new results suggest it might actually find about 400, though we’ll have to wait until Roman starts scanning the sky to make more confident predictions.
Koshimoto says “The microlensing signal from the rogue planet can last from a few hours to about a day, so astronomers will have the opportunity to make simultaneous observations with Roman”.
Scientists will combine Roman’s future data with ground-based observations from facilities such as Japan’s PRIME (Prime-focus Infrared Microlensing Experiment) telescope, located at South Africa’s Sutherland Astronomical Observatory.
This 1.8-meter telescope will build on MOA’s work by performing the first wide-area near-infrared survey of microlenses. It is equipped with four detectors from the Roman detector development program provided by NASA under an international agreement with JAXA (Japan Aerospace Exploration Agency). Each microlensing event is a one-time event, meaning astronomers can’t go back and repeat the observation once it’s over but they are not immediate.