NASA’s Roman mission plans to find 100,000 planets in transit

NASA’s Nancy Grace Roman Space Telescope will create enormous cosmic panoramas, helping us answer questions about the evolution of our universe. Astronomers also expect the mission to find thousands of planets using two different techniques as it studies a wide range of stars in the Milky Way.

Roman will locate these potential new worlds, or exoplanets, by tracking the amount of light coming from distant stars over time. In a technique called a gravitational microlens, a peak of light signals that a planet may be present. On the other hand, if the light of a star periodically decreases, it may be because there is a planet crossing a star’s face as it completes an orbit. This technique is called the transit method. Using these two methods to find new worlds, astronomers will capture an unprecedented view of the composition and arrangement of planetary systems across our galaxy.

Scheduled for launch in the mid-2020s, Roman will be one of NASA’s most prolific planet hunters.

The mission’s large field of view, exquisite resolution, and incredible stability will provide a unique viewing platform to experience the tiny changes in light required to find other worlds through the microlens. This detection method takes advantage of the gravitational light bending effects of massive objects predicted by Einstein’s general theory of relativity.

This happens when a foreground star, the objective, randomly aligns with a distant background star, the source, as seen from Earth. As stars drift in their orbits around the galaxy, the alignment shifts from days to weeks, altering the apparent brightness of the source star. The precise pattern of these changes provides astronomers with clues as to the nature of the foreground lenticular star, including the presence of planets around it.

Many of the stars Roman will already be examining for the microlens survey could harbor planets in transit.

“Microlens events are rare and occur quickly, so you have to look at many stars repeatedly and accurately measure changes in brightness to detect them,” said astrophysicist Benjamin Montet, Senior Lecturer in Scientia at the ‘University of New South Wales in Sydney. “These are the exact same things you need to do to find planets in transit, so by creating a robust microlental study, Roman will produce a nice transit survey as well.”

In a 2017 article, Montet and his colleagues showed that Roman – formerly known as WFIRST – could capture more than 100,000 planets passing by or passing through their host stars. Periodic obscuration when a planet repeatedly passes in front of its star provides strong evidence of its presence, which astronomers usually need to confirm with follow-up observations.

The transit approach to find exoplanets has been very successful for NASA’s Kepler and K2 missions, which have discovered approximately 2,800 confirmed planets to date, and is currently being used by the Transiting Exoplanet Survey Satellite (TESS) of the NASA. Since Roman will find planets orbiting farther and fainter stars, scientists will often have to rely on the mission’s vast dataset to verify the planets. For example, Roman might see secondary eclipses – small dips in brightness when a planetary candidate passes behind his host star, which might help confirm his presence.

The two methods of detecting microlenses and transits complement each other, allowing Roman to find a wide variety of planets. The transit method works best for planets orbiting very close to their star. The microlens, on the other hand, can detect planets orbiting far from their host stars. This technique can also find so-called rogue planets, which are not gravitationally linked to a star at all. These worlds can range from rocky planets smaller than Mars to gas giants.

About three-quarters of the planets in transit that Roman finds are expected to be gas giants like Jupiter and Saturn, or ice giants like Uranus and Neptune. Most of the rest will likely be planets four to eight times the size of Earth, called mini-Neptunes. These worlds are particularly interesting because there are no similar planets in our solar system.

Some of the Roman captures of the worlds in transit are expected to be within their star’s habitable zone, or the range of orbital distances where a planet can harbor liquid water on its surface. The location of this region varies depending on the size and warmth of the host star – the smaller and colder the star, the closer its habitable zone will be. Roman’s sensitivity to infrared light makes him a powerful tool for finding planets around these fainter orange stars.

Roman will also be looking further from Earth than previous planet-hunting missions. Kepler’s original investigation monitored the stars at an average distance of about 2,000 light years. He saw a modest region of the sky, totaling about 115 square degrees. TESS scans almost the entire sky, but it aims to find worlds closer to Earth, with typical distances of around 150 light years. Roman will use both microlens and transit detection methods to find planets up to 26,000 light years away.

Combining the results of Roman’s microlens and searches for transiting planets will help provide a more complete planetary census by revealing worlds with a wide range of sizes and orbits. The mission will provide the first opportunity to find large numbers of transiting planets located thousands of light years away, helping astronomers learn more about the demographics of planets in different regions of the galaxy.

“The fact that we will be able to detect thousands of planets in transit just by looking at the microlens data that have already been taken is exciting,” said Jennifer Yee, study co-author, astrophysicist at the Center for Astrophysics | Harvard & Smithsonian in Cambridge, Massachusetts. “It’s free science.”

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