In 2025, NASA’s next-generation telescope, the Wide-Field Infrared Survey Telescope (WFIRST), will take to space and join in the search for extrasolar planets. Between its 2.4-meter (8 ft) telescope, 18 detectors, 300-megapixel camera, and the extraordinary survey speed it will offer, the WFIRST will be able to scan areas of the sky a hundred times greater than the Hubble Space Telescope.
Beyond its high-sensitivity and advanced suite of instruments, WFIRST will also rely on a technique known as Gravitational Microlensing to search for and characterize exoplanets. This is essentially a small-scale version of the gravitational lensing technique, where the gravitational force of a massive object between the observer and the target is used to focus and magnify the light coming from a distant source.
To date, the majority of exoplanets have been discovered using the Transit Method (aka. Transit Photometry), where the passage of a planet in front of a star (i.e. a transit) results in periodic dips in brightness that can be measured. While WFIRST will monitor for these dips in brightness as well, it will also be watching for periodic surges in radiance produced by microlensing events.
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The Microlensing Method
Like Gravitational Lensing (GL), these events were predicted by Einstein’s General Theory of Relativity, which states that the curvature of spacetime becomes altered in the presence of the gravitational force generated by a massive object. Whereas GL relies on galaxies and galaxy clusters, microlensing relies on chance alignments between two distant stars as they drift through space.
Whenever two stars align closely from our vantage point here on Earth, the light from the more distant star curves as it travels past the warped space-time of the nearer star. If the alignment is especially close, the nearer stat will have a “lensing” effect where it magnifies the light coming from the background star. Similarly, planets that orbit a foreground star have a small lensing effect as well that will reveal much about the planet itself.
Because of how they rely on change alignments, microlensing events are a rare occurrence compared to transits. But with its ability to scan a much wider area of the sky than any previous space telescope, WFIRST will have a much better chance of detecting these events. As David Bennett, who leads the gravitational microlensing group at NASA’s Goddard Space Flight Center, explained:
“Microlensing signals from small planets are rare and brief, but they’re stronger than the signals from other methods. Since it’s a one-in-a-million event, the key to WFIRST finding low-mass planets is to search hundreds of millions of stars.”
Moreover, microlensing is a better method for finding planets that orbit within and beyond their respective star’s habitable zones (HZs). For instance, microlensing events caused by orbiting planets can allow astronomers to place tight constraints on a planet’s mass and distance from its host star – whereas the Transit Method is good for gauging a planet’s size and orbital period, but not its mass or distance.
Combined with the many discoveries made by NASA’s Kepler and Transiting Exoplanet Survey Satellite (TESS) missions, WFIRST will complete the first census containing exoplanets with a wide range of masses and orbits, allowing astronomers to narrow the search for habitable worlds. Said Matthew Penny, an assistant professor at Louisiana State University who led a study to predict WFIRST’s microlensing survey capabilities:
“Trying to interpret planet populations today is like trying to interpret a picture with half of it covered. 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 WFIRST’s microlensing survey, combined with the results from Kepler and TESS, will reveal far more of the picture.
“WFIRST’s microlensing survey will not only advance our understanding of planetary systems, 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.”
Unlike previous surveys, WFIRST will explore regions of the galaxy that haven’t been systematically searched yet – which includes the center of our galaxy, where the majority of stars in the Milky Way reside. Because of its infrared imaging capabilities, WFIRST will be able to see through the obscuring gas and dust that prevent telescopes from studying planets in the crowded central region.
On top of that, WFIRST’s surveys will also be a lot more ambitious than its predecessors. Between 2009 and 2018, Kepler searched 100 square degrees of the sky and tracked 100,000 stars at a distance of about 1000 light-years. Currently, TESS is scanning the entire sky and tracking 200,000 stars at a distance of about 100 light-years.
WFIRST will search an area of the sky measuring just 3 square degrees but will follow 200 million stars at distances of around 10,000 light-years. Combined with the James Webb Space Telescope (JWST), another next-generation infrared observatory, the study of exoplanets is about to go into hyperdrive!
Further Reading: NASA