Utilising
NASA’s Spitzer Space Telescope, astronomers have discovered a distant
exoplanet located 13,000 light-years away, through gravitational
microlensing. The red cone in the map above our Solar System, depicts
the extend in the galaxy of the thousands of known exoplanets that have
been discovered by NASA’s Kepler space telescope, while the red circle
represents the extend of all the exoplanets that have been detected so
far by ground-based telescopes. The white dots show the positions of
exoplanets that have been discovered through the gravitational
microlensing technique. Image Credit: NASA/JPL-Caltech
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.
The 1.3-m Warsaw University Telescope used by the OGLE survey. Image Credit: OGLE
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.
Infographic
explaining how the Spitzer Space Telescope can be used in tandem with a
ground-based telescope, in order to measure the distances to exoplanets
discovered using gravitational microlensing. Image Credit:
NASA/JPL-Caltech/Warsaw University University
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 towards the direction of the Milky Way’s central bulge, while the star-planet separation was estimated to be about 3.1 AU.
This
plot shows the light curve of OGLE-2014-BLG-0124L, obtained from NASA’s
Spitzer Space Telescope and the OGLE survey, during the summer of 2014.
The finding was the result of fortuitous timing because Spitzer’s
overall program to observe microlensing events was only just starting up
in the week before the planet’s effects were visible from Spitzers
vantage point. Image Credit: NASA/JPL-Caltech/Warsaw University
University
The
proposed WFIRST-AFTA mission will greatly complement any present and
future planet-hunting missions, by allowing astronomers to map the
distribution of exoplanets throughout the galaxy. Image Credit:
NASA/WFIRST/Matthew Penny (Ohio State University)
“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 2020’s will truly constitute a new golden era in exoplanetary research, bearing discoveries that are unimaginable today.