In the fictional universe of Star Trek: Voyager, the crew of a Federation starship comes in contact with the denizens of planetary systems many thousands of light-years away from Earth, while being stranded on the far side of the galaxy. In an example of life imitating art, astronomers have been able to discover dozens of exoplanets across the Milky Way in recent years, through the use of gravitational microlensing. A research team has recently added one more member to the list, by announcing the detection of an exoplanet at a distance of approximately 13,000 light-years away, which was spotted by NASA’s Spitzer space telescope in conjunction with a ground-based, deep-sky survey.
As described in a previous AmericaSpace article, gravitational lensing is an effect of the curvature of space-time by gravity that was first described by Einstein’s theory of General Relativity in the early 20th century and can be described as the phenomenon of the bending of light of distant, faraway cosmic sources (like quasars and other distant galaxies) from the gravity of massive objects (like galaxy clusters) that lie in between. The gravity of these intermediate objects bends and refocuses the light of the more distant sources, acting like a lens which brightens and magnifies the latter, thus allowing us to observe distant parts of the Universe that would otherwise be beyond our view. Gravitational microlensing, on the other hand, results from the bending of light from much smaller and less massive stellar-type objects like brown dwarfs, red dwarfs, neutron stars, and black holes. Because the mass and size of the latter is many orders of magnitude smaller compared to that of galaxies, they brighten the light of passing background objects significantly less, making them much more challenging for astronomers to detect.
Despite these observational challenges, astronomers have successfully spotted many thousands of such microlensing events as part of various comprehensive deep-sky surveys during the last couple decades which have monitored hundreds of millions of stars for many years at a time, like the MACHO Collaboration project, the Microlensing Observations in Astrophysics, or MOA, and the Optical Gravitational Lensing Experiment, or OGLE. These have provided great advances to many important areas of astrophysical research, like the study for the nature of dark matter in the halos of the Milky Way and its neighboring galaxies, the characterisation of thousands of variable stars, and the search for exoplanets.
Contrary to the radial velocity and transit methods which are more widely used for exoplanet discovery, gravitational microlensing can only be used when the light from a distant, background star is magnified by the gravitational field of a closer, foreground star that happens to pass in front, as seen by our line of sight here on Earth. In the case of two stars without planets, the background star’s brightness will increase as the foreground star passes in front of it and then decrease as the latter moves away, in a predictable way during a period of days or weeks, producing a well-defined light curve. If the foreground star happens to have any planets orbiting it, these will distort and dim the light from the background star in a noticeable way as well, which will help astronomers measure some of their basic properties, like their mass and orbital period. And since both stars must be exactly aligned for this method to work, such exoplanet microlensing events are extremely rare. Nevertheless, astronomers have been able to detect dozens of microlensing exoplanets, proving that this is a viable and important method for the discovery and characterisation of other planetary systems in the galaxy. The OGLE survey, which utilises the 1.3-m Warsaw University Telescope located at the Las Campanas Observatory in Chile, has been at the forefront of this research, by discovering, among other things, the most distant known exoplanet to date in the Milky Way, located at a distance of more than 22,000 light-years, very near the galactic center, while also detecting a candidate planet-like object inside our neighboring Andromeda galaxy, which, if confirmed, would be the first discovery ever of a planet outside of our galaxy.
Now, an international team of astronomers, led by Jennifer Yee from the Harvard-Smithsonian Center for Astrophysics and Andrzej Udalski from the Warsaw University Astronomical Observatory, has announced the discovery of an extrasolar world at a distance of 13,000 light-years away, toward the direction of the Milky Way’s center, while utilising OGLE simultaneously with NASA’s Spitzer Space Telescope. More specifically, the researchers sought out to determine whether NASA’s infrared orbiting observatory could be used to make space-based parallax measurements of microlensing events soon after they had been recorded from ground-based telescopes. Parallax is a standard method in astronomy for measuring the distance of nearby stars. By observing the shift in the relative positions of stars in the sky relative to Earth as the latter moves in its orbit around the Sun, astronomers can triangulate their distance with great accuracy.
Similarly, Yee’s team used Spitzer throughout the summer of 2014 for an 100-hour pilot observing program, during which they studied a microlensing event of interest that had been previously detected by the OGLE survey in February. By taking advantage of Spizer’s large distance from the Earth, the researchers were able to observe the light curve of the event from the vantage point of the orbiting telescope and study its variations with time in order to check them against similar observations that were conducted at the same time with the OGLE telescope on Earth. Through this process, the astronomers eventually were able to determine that the event was caused by the magnifying of a single star’s light due to the foreground passage of an orbiting planet-type object with a mass of approximately 0.5 times that of Jupiter. Consequently, these observations of the same microlensing event from two different largely separated vantage points allowed Yee’s team to triangulate the distance of the newly discovered planet, named OGLE-2014-BLG-0939L, determining that it was approximately 13,000 light-years away, toward the direction of the Milky Way’s central bulge, while the star-planet separation was estimated to be about 3.1 AU.
One significant aspect of this discovery is the fact that it’s the first of its kind to be made by an orbiting space telescope. “Spitzer is the first space telescope to make a microlens parallax measurement for a planet,” says Yee. “Traditional parallax techniques that employ ground-based telescopes are not as effective at such great distances.”
Furthermore, gravitational microlensing can complement other exoplanet detection techniques like radial velocity and the transit method, which are limited in discovering mostly massive planets in relatively close orbits around their host stars. In addition, the bulk of the thousands of exoplanets that have been discovered to date have been found within a radius of a few thousand light-years from Earth. Gravitational microlensing opens up the possibility of mapping the vast expanses of the entirety of the Milky Way to really study the distribution of planets across the galaxy and discover planet-type objects that can’t be detected with any of the other planet-hunting methods that are currently being used. “There are several major benefits to such a study,” write’s Yee’s team in its study, which appeared in the April 1 issue of The Astrophysical Journal. “First, it is the only way to obtain a mass-based census of stellar, remnant, and planetary populations. Several components of this population are dark or essentially dark including free-floating planets, brown dwarfs, neutron stars, and black holes and therefore are essentially undetectable by any other method unless they are orbiting other objects. In addition, even the luminous-star mass function of distant populations (e.g., in the Galactic Bulge) is substantially more difficult to study photometrically than is generally imagined. For example, a large fraction of stars are fainter components in binary systems, with separations that are too small to be separately resolved, but whose periods are too long (or primaries too faint) for study by the radial velocity technique.”
“We’ve mainly explored our own solar neighborhood so far,” adds Sebastiano Calchi Novati, a Visiting Sagan Fellow at NASA’s Exoplanet Science Institute at the California Institute of Technology in Pasadena, Calif., and co-author of the study. “Now we can use these single lenses to do statistics on planets as a whole and learn about their distribution in the galaxy.”
Bearing that in mind, the latest Astronomy and Astrophysics Decadal Survey by the National Research Council in 2010 acknowledged the proposed Discovery-class Wide Field Infrared Survey Telescope-Astrophysics Focused Telescope Assets mission, or WFIRST-AFTA for short, as a top priority for the 2020s. One of the main science objectives of WFIRST-AFTA mission proposal, which calls for using one of the two 2.4-m telescopes that were donated to NASA by the National Reconnaissance Office in 2012, is to provide a complete census of exoplanets throughout the galaxy through the use of gravitational microlensing. “WFIRST addresses fundamental and pressing scientific questions and will contribute to a broad range of astrophysics,” wrote the NRC’s Astro2010 Decadal Survey. “It complements the committee’s proposed ground-based program in two key science areas: dark energy science and the study of exoplanets.” If WFIRST-AFTA gets the final go-ahead by NASA for implementation, it could open up a new chapter in exoplanet research as a follow-up to the science results by ESA’s Gaia space observatory which has already began its primary five-year mission, as well as the James Webb Space Telescope which is scheduled for launch in 2018.
“We don’t know if planets are more common in our galaxy’s central bulge or the disk of the galaxy, which is why these observations are so important,” says Yee. Provided that NASA goes forward with the WFIRST-AFTA mission proposal and the space agency receives the funding necessary for the next round of Discovery-class missions, the 2020s will truly constitute a new golden era in exoplanetary research, bearing discoveries that are unimaginable today.