Nov 5 2019
Telescopes across the globe have been used by scientists to confirm and characterize an exoplanet orbiting an adjacent star, by means of a rare phenomenon called gravitational microlensing.
Diagram illustrating the microlensing event studied in this research. Red dots indicate previous exoplanet systems discovered by microlensing. Inset: Artist’s conception of the exoplanet and its host star. (Image credit: The University of Tokyo)
Although the mass of the exoplanet is similar to that of Neptune, it orbits a star lighter (cooler) than the Sun at an orbital radius analogous to that of Earth.
This orbital region surrounds cool stars and is considered to be the place of origin of gas-giant planets. The study outcomes indicate that Neptune-sized planets could be more prevalent around this orbital region.
Since the exoplanet found this time is closer compared to other exoplanets found with the same technique, it is an ideal target for follow-up observations using world-leading telescopes, such as the Subaru Telescope.
Tadashi Kojima, an amateur astronomer, reported the sighting of a mysterious new object in the Taurus constellation, from the Gunma Prefecture in Japan, on November 1st, 2017. Astronomers across the globe started follow-up observations and established that this was an example of a rare event called gravitational microlensing.
According to Einstein’s Theory of General Relativity, gravity warps space. In outer space, when a foreground object possessing strong gravity moves directly in front of a background object, this warped space can serve as a lens and focus the background object’s light, which makes it seem brighter momentarily.
With respect to the object detected by Kojima, a star located 1600 light-years away moves in front of a star located 2600 light-years away. Moreover, by analyzing the variation in the lensed brightness, astronomers identified that a planet was orbiting the foreground star.
Using the microlensing technique, exoplanets have been discovered even before. However, microlensing events are short-lived and rare; therefore, the events identified to date lie toward the Galactic Center, at which the stars are most abundant. By contrast, this exoplanet system was discovered in virtually precisely the opposite direction as detected from the Earth.
A group headed by Akihiko Fukui from the University of Tokyo used a collection of 13 telescopes positioned across the globe, including the 91-cm telescope and 188-cm telescope at NAOJ’s Okayama Astrophysical Observatory, spotted this phenomenon for 76 days and gathered adequate data to ascertain the properties of the exoplanet system.
The mass of the host star is almost half of that of the Sun. The size of the orbit of the exoplanet around the host star is similar to that of Earth’s orbit, where the exoplanet is nearly 20% heavier than Neptune.
Such an orbital radius around a star of this type coincides with the region in which water condenses into ice at the time of the planet formation phase, rendering this place theoretically favorable for the formation of gas-giant planets.
As per theoretical calculations, planets of this kind have an a priori detection probability of just 35%. The fact that this exoplanet was spotted purely by luck indicates that Neptune-sized planets could be prevalent around this orbital region.
When observed from Earth, this exoplanet system is brighter and closer than other exoplanet systems spotted through microlensing, making it a principal target for follow-up observations with world-class telescopes, such as the Subaru Telescope, or next-generation extremely large telescopes, such as the Thirty Meter Telescope (TMT).
The outcomes of this study have been published as Fukui et al., “Kojima-1Lb is a Mildly Cold Neptune around the Brightest Microlensing Host Star” in The Astronomical Journal on November 1st, 2019.