Our Universe is full of exotic objects and phenomena, and we have built telescopes and satellites to try and attain a deep understanding of it all. Sometimes, objects in space come to our aid and they act like magnifying lenses to provide us a glance at other faint (very far away) sources. In astronomy, this phenomenon is called Gravitational Lensing, which states that when one massive object (“foreground source”) passes in front of another (“background source”) across our line of sight, it bends the light coming from this background source and we end up seeing multiple magnified images of this far-away source. This bending of light occurs due to the gravitational field associated with the foreground source, which distorts the light coming from the background object. The foreground source is the “lensing object” and it can be a star, a galaxy, a cluster of galaxies, or black holes, etc. The more massive the lens is, the stronger is the lensing effect because of its strong gravitational field.
Astronomers found that this lensing object can also be an exoplanet, which is less massive as compared to other objects in space but can create the lens effect on a faraway background star. In this case, this phenomenon is called microlensing. Astronomers have been able to find more than 50 exoplanets using this method. Moreover, we can also estimate the mass of these lenses provided we know two parameters: (1) the size of the theoretical Einstein ring (for perfect alignment, the background star would look like a ring image around the lensing object.) and (2) parallax ( an angle formed at the object when viewed from two different angles). Using these two parameters, one can find the mass of this lens. The first parameter is not that hard to obtain, but the second one is not always available.
For the first time, astronomers from France, Poland, Belgium, and the USA collaborated to witness the lensing effect in real time where they witness the background star’s image in motion as well as rotation, all thanks to the lensing effect by an unknown (dark) foreground object. They used the Very Large Telescope Interferometer (VLTI) along with many other telescopes from the ground as well as the Spitzer Space Telescope in the sky to obtain real-time observations of this effect on a star called Gaia19bld. This collaboration of various telescopes as well as precise measurements from VLTI allowed this team to calculate the mass of the dark lens to be =1.147 Msun. The nature of this object however is still uncertain. These results were published in the highly regarded peer-reviewed journal, Nature, in December 2021.