Named after Nancy Roman, known as the “Mother of Hubble,” NASA’s Nancy Grace Roman Telescope will explore uncharted regions of the galaxy for exoplanets, focusing on star-systems toward the crowded, chaotic center of our Milky Way galaxy. Studying the population properties of exoplanets will help us understand what planetary systems throughout the galaxy are like and how planets form and evolve.
Previous missions such as NASA’s Kepler space telescope and K2 mission, searched for exoplanets in a modest-sized region of about 100 square degrees on the sky with 100,000 stars at typical distances of around a thousand light-years. NASA’s Transiting Exoplanet Survey Satellite (TESS) is currently scannings the entire sky for exoplanets. TESS tracks hundreds of thousands of stars; however their typical distances are around 100 light-years. The Roman telescope will search roughly 3 square degrees of the sky, but will observe 200 million stars at distances out to 10,000 light-years.
Discovering Alien Star Systems
The Roman telescope will search for planets outside our solar system toward the center of our Milky Way galaxy, where most stars are. Studying the population properties of exoplanets will help us understand what planetary systems throughout the galaxy are like and how planets form and evolve.
Combining Roman’s findings with results from NASA’s Kepler and TESS missions will complete the first planet census that is sensitive to a wide range of planet masses and orbits, bringing us a step closer to discovering habitable Earth-like worlds beyond our own.
To date, astronomers have found most planets when they pass in front of their host star in events called transits, which temporarily dim the star’s light. Roman telescope data can spot transits, too, but the mission will primarily watch for the opposite effect – little surges of radiance produced by a light-bending phenomenon called microlensing. These events are much less common than transits because they rely on the chance alignment of two widely separated and unrelated stars drifting through space.
“Microlensing signals from small planets are rare and brief, but they’re stronger than the signals from other methods,” said David Bennett, who leads the gravitational microlensing group at NASA’s Goddard Space Flight Center. “Since it’s a one-in-a-million event, the key to finding low-mass planets is to search hundreds of millions of stars.”
The microlensing technique is better at finding planets in and beyond the habitable zone – the orbital distances where planets might have liquid water on their surfaces. This effect occurs when light passes near a massive object. Anything with mass warps the fabric of space-time, sort of like the dent a bowling ball makes when set on a trampoline. Light travels in a straight line, but if space-time is bent – which happens near something massive, like a star – light follows the curve.
Star acts like a natural cosmic lens
Any time two stars align closely from our vantage point, light from the more distant star curves as it travels through the warped space-time of the nearer star. This phenomenon, one of the predictions of Einstein’s general theory of relativity, was famously confirmed by British physicist Sir Arthur Eddington during a total solar eclipse in 1919. If the alignment is especially close, the nearer star acts like a natural cosmic lens, focusing and intensifying light from the background star.
Planets orbiting the foreground star may also modify the lensed light, acting as their own tiny lenses. The distortion they create allows astronomers to measure the planet’s mass and distance from its host star. This is how Roman will use microlensing to discover new worlds.
“Trying to interpret planet populations today is like trying to interpret a picture with half of it covered,” said Matthew Penny, an assistant professor of physics and astronomy at Louisiana State University in Baton Rouge who led a study to predict the Roman telescope’s microlensing survey capabilities. “To fully understand how planetary systems form we need to find planets of all masses at all distances. No one technique can do this, but Roman’s microlensing survey, combined with the results from Kepler and TESS, will reveal far more of the picture.”
More than 4,000 exoplanets have been confirmed so far, but only 86 were found via microlensing. The techniques commonly used to find other worlds are biased toward planets that tend to be very different from those in our solar system. The transit method, for example, is best at finding sub-Neptune-like planets that have orbits much smaller than Mercury’s. For a solar system like our own, transit studies could miss every planet.
Will find analogs to every planet in our solar system
Roman’s microlensing survey will help us find analogs to every planet in our solar system except Mercury, whose small orbit and low mass combine to put it beyond the mission’s reach. The Roman telesscope will find planets that are the mass of Earth and even smaller – perhaps even large moons, like Jupiter’s moon Ganymede.
Roman will find planets in other poorly studied categories, too. Microlensing is best suited to finding worlds from the habitable zone of their star and farther out. This includes ice giants, like Uranus and Neptune in our solar system, and even rogue planets – worlds freely roaming the galaxy unbound to any stars.
While ice giants are a minority in our solar system, a 2016 study indicated that they may be the most common kind of planet throughout the galaxy. Roman will put that theory to the test and help us get a better understanding of which planetary characteristics are most prevalent.
Hidden Gems in the Galactic Core
Since Roman is an infrared telescope, it will see right through the clouds of dust that block other telescopes from studying planets in the crowded central region of our galaxy. Most ground-based microlensing observations to date have been in visible light, making the center of the galaxy largely uncharted exoplanet territory. A microlensing survey conducted since 2015 using the United Kingdom Infrared Telescope (UKIRT) in Hawaii is smoothing the way for Roman’s exoplanet census by mapping the region.
The UKIRT survey is providing the first measurements of the rate of microlensing events toward the galaxy’s core, where stars are most densely concentrated. The results will help astronomers select the final observing strategy for WFIRST’s microlensing effort.
The UKIRT team’s most recent goal is detecting microlensing events using machine learning, which will be vital for WFIRST. The mission will produce such a vast amount of data that combing through it solely by eye will be impractical. Streamlining the search will require automated processes.
“Our current survey with UKIRT is laying the groundwork so that Roman can implement the first space-based dedicated microlensing survey,” said Savannah Jacklin, an astronomer at Vanderbilt University in Nashville, Tennessee, who has led several UKIRT studies. “Previous exoplanet missions expanded our knowledge of planetary systems, and Roman will move us a giant step closer to truly understanding how planets – particularly those within the habitable zones of their host stars – form and evolve.”
Detecting Bizarre Cosmic Objects
The same microlensing survey that will reveal thousands of planets will also detect hundreds of other bizarre and interesting cosmic objects. Scientists will be able to study free-floating bodies with masses ranging from that of Mars to 100 times the Sun’s.
The low end of the mass range includes planets that were ejected from their host stars and now roam the galaxy as rogue planets. Next are brown dwarfs, which are too massive to be characterized as planets but not quite massive enough to ignite as stars. Brown dwarfs don’t shine bright in visible light the way stars do, but Roman will be able to study them in infrared light where they give off the majority of their heat.
Objects at the higher end include stellar corpses – neutron stars and black holes – left behind when massive stars exhaust their fuel. Studying them and measuring their masses will help scientists understand more about stars’ death throes while providing a census of stellar-mass black holes.
“Roman’s microlensing survey will not only advance our understanding of planetary systems,” said Penny, “it will also enable a whole host of other studies of the variability of 200 million stars, the structure and formation of the inner Milky Way, and the population of black holes and other dark, compact objects that are hard or impossible to study in any other way.”
The Roman telescope is managed at Goddard, with participation by NASA’s Jet Propulsion Laboratory and Caltech/IPAC in Pasadena, the Space Telescope Science Institute in Baltimore, and a team comprising scientists from research institutions across the United States.
Image at the top of the page shows the central region of the Milky Way that contains an exotic collection of objects, including a supermassive black hole weighing about 4 million times the mass of the Sun (called Sagittarius A*), clouds of gas at temperatures of millions of degrees, neutron stars and white dwarf stars tearing material from companion stars and beautiful tendrils of radio emission.
The region around Sagittarius A* is shown in this new composite image at top of the page with Chandra data (green and blue) combined with radio data (red) from the MeerKAT telescope in South Africa, which will eventually become part of the Square Kilometer Array. (X-Ray:NASA/CXC/UMass/D. Wang et al.; Radio:NRF/SARAO/MeerKAT
Editor, Jackie Faherty, astrophysicist, Senior Scientist with AMNH. Jackie was formerly a NASA Hubble Fellow at the Carnegie Institution for Science. Aside from a love of scientific research, she is a passionate educator and can often be found giving public lectures in the Hayden Planetarium. Her research team has won multiple grants from NASA, NSF, and the Heising Simons foundation to support projects focused on characterising planet-like objects. She has also co-founded the popular citizen science project entitled Backyard Worlds: Planet 9 which invites the general public to help scan the solar neighbourhood for previously missed cold worlds. A Google Scholar, Faherty has over 100 peer reviewed articles in astrophysical journals and has been an invited speaker at universities and conferences across the globe. Jackie received the 2020 Vera Rubin Early Career Prize from the American Astronomical Society, an award that recognises scientists who have made an impact in the field of dynamical astronomy and the 2021 Robert H Goddard Award for science accomplishments.