Since the 1990s, astronomers have discovered more than 4,000 exoplanets – planets outside our solar system, orbiting other stars. NASA’s upcoming Roman Space Telescope – named for American astronomer Nancy Grace Roman – is one of the next generation of space telescopes that will play a big part in discovering more new worlds. In fact, NASA is expecting Roman, which is scheduled to launch in the mid-2020s, to discover upwards of 100,000 exoplanets, the space agency announced on March 31, 2021.
How will Roman do this? The telescope will use two different methods for detecting exoplanets, the transit method and microlensing. Most telescopes use primarily just one method, but by using two different ones, Roman – previously known as the Wide Field Infrared Survey Telescope (WFIRST) – will be one of the most prolific planet hunters ever launched.
Microlensing uses the gravitational light-bending effects of massive objects to detect planets orbiting a star. It does this by monitoring the tiny changes in light produced by the star. This happens when a more distant star happens to align with the closer foreground star, the one that is being studied for the presence of possible planets. The closer star acts as a sort of lens, bending the light coming from the further star. As the alignment changes slightly over days and weeks, due to the movement of the stars, the brightness of the more distant star changes slightly also. By looking at the pattern of changes in the light of the closer target star, astronomers can find clues as to there are any planets orbiting it.
Wide, foil-covered open-ended cylinder in space with stars in background.
The Nancy Grace Roman Space Telescope, as seen in this artist’s illustration. Image via NASA/ GSFC/ Hubblesite.
This kind of microlensing event doesn’t happen often, however, so Roman will also search for planets using the most common technique, the transit method. As Ben Montet, a Scientia Lecturer at the University of New South Wales, said in a statement:
Microlensing events are rare and occur quickly, so you need to look at a lot of stars repeatedly and precisely measure brightness changes to detect them. Those are exactly the same things you need to do to find transiting planets, so by creating a robust microlensing survey, Roman will produce a nice transit survey as well.
Most of Roman’s discoveries should come from the transit method. Montet says that the telescope should be able to find 100,000 planets, or more, transiting in front of their stars. Montet previously published a paper in 2017 detailing what Roman should be able to accomplish, when it was called WFIRST.
In such transits, the brightness of the host star dims very slightly as a planets transits in front of it, as seen from Earth. By measuring the changes in light, scientists can determine to some accuracy how large a planet is. Follow-up observations can often help determine the mass of the planet and how large its orbit is.
While the transit method is expected to find many more planets than microlensing, both methods are complementary to each other. Many exoplanets have been found orbiting very close to their stars, such as hot Jupiters, and the transit method is better suited for finding those kinds of planets. Microlensing, on the other hand, is more useful for detecting planets orbiting farther away from their stars. It can even find rogue planets, ones that don’t orbit any star at all, and are just floating around freely in interstellar space! The combination of the two methods will also help scientists find exoplanets with a wide variety of sizes and orbits.
According to Jennifer Yee, an astrophysicist at the Center for Astrophysics | Harvard & Smithsonian:
The fact that we’ll be able to detect thousands of transiting planets just by looking at microlensing data that’s already been taken is exciting. It’s free science.
What kinds of planets will Roman discover?
Most, about 3/4, will probably be gas giants similar to Jupiter and Saturn. Also ice giant type worlds, like Uranus and Neptune, are expected to be among those. Most of the rest may be mini-Neptunes, four to eight times the mass of Earth. Mini-Neptunes are a bit larger than super-Earths, which are a little larger and more massive than Earth.
Milky Way galaxy with green and blue beams coming from, and a small red circle around, the location of Earth, with text.
Comparison of exoplanet search areas for three different space telescopes: Roman space telescope, TESS and Kepler. Roman will peer deeper into our galaxy than any previous mission. Image via NASA/ Goddard Space Flight Center.
Some of these planets will likely be within the habitable zones of their stars, the region where temperatures could allow water to remain liquid on the surfaces of rocky worlds.
Roman will cover new celestial territory and look deeper into our galaxy than ever before. It will be able to find planets up to 26,000 light-years away. By comparison, the Kepler Space Telescope, which has now finished its mission, studied stars up to 2,000 light-years distant on average, and TESS currently focuses on looking for planets up to about 150 light-years away.
Missions like Kepler and TESS, as well as Earth-based telescopes, have already discovered a wide variety of worlds in the small regions of our galaxy examined so far. These range from rocky planets about the size of Earth and a bit smaller, to gas giant worlds larger than Jupiter. It will be exciting to see what kinds of new worlds that Roman finds.
Smiling man with eyeglasses and beard and green shirt.
In 2017, Ben Montet at the University of New South Wales published a paper outlining what Roman will be able to accomplish. Image via UNSW.
Roman was named after Nancy Grace Roman (1925-2018), NASA’s first chief astronomer, who paved the way for space telescopes focused on studying the broader universe. She is also considered to be the “mother” of the beloved Hubble Space Telescope. Thomas Zurbuchen, NASA’s associate administrator for science, said:
Nancy Grace Roman was a leader and advocate whose dedication contributed to NASA seriously pursuing the field of astrophysics and taking it to new heights. Her name deserves a place in the heavens she studied and opened for so many.