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Microlensing ideal for finding Neptune-sized exoplanets, study shows

Neptune-mass exoplanets like the one shown in this artist's rendering may be the most common in the icy regions of planetary systems. Beyond a certain distance from a young star, water and other substances remain frozen, leading to an abundant population of icy objects that can collide and form the cores of new planets. In the foreground, an icy body left over from this period drifts past the planet. Credits: NASA/Goddard/Francis Reddy

Neptune-mass exoplanets like the one shown in this artist’s rendering may be the most common in the icy regions of planetary systems. Image Credit: Francis Reddy / NASA

Gravitational microlensing, one of several techniques used to search for exoplanets, is the ideal method for finding Neptune-sized planets in distant orbits around their stars, according to a study published in the Dec. 13, 2016, issue of The Astrophysical Journal.

Gas giants with masses and sizes similar to those of Neptune in the Solar System appear to be the most abundant type of planets orbiting in the outer regions of other star systems.

Based on the gravitational lens effect, predicted by Einstein’s theory of relativity, gravitational microlensing works when a foreground object, such as a star, aligns with a more distant object. The foreground object bends the light of the background object, producing multiple magnified, and at times distorted, images of the latter object.

An artist's rendering of NASA's Wide Field Infrared Survey Telescope (WFIRST).

An artist’s rendering of NASA’s Wide-Field Infrared Survey Telescope (WFIRST). Image Credit: NASA/GSFC/Conceptual Image Lab

Over time, the alignment of the two objects changes as the foreground, or lensing, star moves in its orbit around the galactic center. This movement produces changes in the brightness of the background star from which astronomers can obtain details about the foreground star, including planets orbiting it.

While several techniques, including radial velocity and detection of a host star’s dimming, are used in exoplanet searches, microlensing is most effective for finding planets in distant orbits around their stars, rogue planets that do not orbit any stars, and low-mass planets orbiting faint stars.

David Bennett, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who took part in the study, said that if one or more exoplanets are detected, they mainly determine the mass ratio of the planet to the host star and their separation.

“For about 40 percent of microlensing planets, we can determine the mass of the host star and therefore the mass of the planet,” Bennett said.

Searching with WFIRST


NASA’s Wide Field Infrared Survey Telescope (WFIRST), set to launch sometime during the mid-2020s, will use microlensing to search for exoplanets as well as determine their masses and distances from their stars.

It will characterize exoplanets through the use of coronagraphy, which blocks the light of the star, enabling observation of its planets.

With a combination of high resolution and a very large field of view, WFIRST will search for microlensing events in the direction of the galactic center. It will be capable of finding exoplanets with masses as low as that of Mars in stellar orbits ranging from that of Earth to beyond that of Neptune.

Data from WFIRST will give scientists the most comprehensive view yet of the formation and evolution of planetary systems; plus, it will serve as a crucial step toward finding a habitable Earth-like planet.

NASA’s Kepler and K2 missions, which have discovered more than 2,500 exoplanets, use the dimming of host stars method, which is ideal for planets in close orbits but much less effective for those in distant orbits.

Challenges


This graph plots 4,769 exoplanets and planet candidates according to their masses and relative distances from the snow line, the point where water and other materials freeze solid (vertical cyan line). Gravitational microlensing is particularly sensitive to planets in this region. Planets are shaded according to the discovery technique listed at right. Masses for unconfirmed planetary candidates from NASA's Kepler mission are calculated based on their sizes. For comparison, the graph also includes the planets of our solar system. Caption and Image Credit: NASA

This graph plots 4,769 exoplanets and planet candidates according to their masses and relative distances from the snow line, the point where water and other materials freeze solid (vertical cyan line). Gravitational microlensing is particularly sensitive to planets in this region. Planets are shaded according to the discovery technique listed at right. Masses for unconfirmed planetary candidates from NASA’s Kepler mission are calculated based on their sizes. For comparison, the graph also includes the planets of our solar system. (Click to enlarge) Image & Caption Credit: NASA

Microlensing presents challenges of its own. Slightly more than 50 exoplanets have been discovered via this technique, as compared with thousands found through other methods. It requires observers to spend significant time looking for the necessary alignment of two stars needed for a microlensing event.

Despite this, scientists anticipate the method will have tremendous potential for finding planets hundreds of times more distant than any found using other techniques, making it possible to extend exoplanet searching to a larger portion of the galaxy.

Several microlensing projects have already been conducted or are underway. Between 2007 and 2012, researchers in Japan and New Zealand worked together on the Microlensing Observations in Astrophysics (MOA) project, which kept track of microlensing events for astronomers.

The project, which incorporated data from several other microlensing efforts, such as the Optical Gravitational Lensing Experiment (OGLE), resulted in the discovery of four new exoplanets.

Neptune-sized planets


From measuring the prevalence of planets compared to the mass ratio of planets to their stars, as well as the distances between planets and their host stars, MOA researchers found the most common type of planets orbiting stars of approximately 60 percent of the Sun’s mass are gas giants about the size and mass of Neptune.

Typical planets found via microlensing have masses ranging from 10 to 40 times that of Earth. Neptune is about 17 Earth masses.

“We’ve found the apparent sweet spot in the sizes of cold planets,” noted study lead scientist Daisuke Suzuki of Goddard and of the University of Maryland.

“Contrary to some theoretical predictions, we infer from current detections that the most numerous have masses similar to Neptune, and there doesn’t seem to be the expected increase in number at lower masses. We conclude that Neptune-mass planets in these outer orbits are about 10 times more common than Jupiter-mass planets in Jupiter-like orbits.”

Neptune-sized planets appear to be the most common in regions beyond what is known as the snow line, the location where water stayed frozen throughout the time planets formed.

In the Solar System, that line is about 2.7 times Earth’s distance from the Sun.

“Beyond the snow line, materials that were gaseous closer to the star condense into solid bodies, increasing the amount of material available to start the planet-building process,” Suzuki said. “This is where we think planetary formation was most efficient, and it’s also the region where microlensing is most sensitive.”

Video courtesy of NASA Goddard

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Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.

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