NASA’s exoplanet-hunting spacecraft TESS (Transiting Exoplanet Survey Satellite) has a new method for detecting worlds beyond the solar system. The technique relies on a phenomenon introduced by Einstein in his 1915 theory of gravity, general relativity, called gravitational microlensing.
The exoplanet in question is called Gaia23bra b. The first hints of this exoplanet were found in 2023 by the now-retired Gaia space telescope via the slight brightening of a star caused by a microlensing event.
TESS usually spots planets by the tiny drop in the light output from their parent star as they cross, or transit, its face. This technique is most effective for very large gas giants that orbit close to their star, so it most likely wouldn’t work for Gaia23bra b, which has 1.6 times Jupiter’s mass but orbits its star at a similar distance to Jupiter’s orbit around the sun. Additionally, the transit method employed by TESS usually has a search radius of around 150 light-years. Gaia23bra b, however, orbits an orange dwarf star about 80 percent the size of the sun that is located 40,000 light-years away. Thus, to confirm the existence of this world, TESS had to learn a new trick.
“When TESS launched, no one expected it to ever be capable of finding this kind of planet,” team member Diana Dragomir of the University of New Mexico said in a statement. “The discovery implies that there are probably other so-called microlensing planets hiding in TESS’s data that we hadn’t previously thought to look for.”
Microlensing and the hunt for exoplanets
To understand what microlensing is, first we have to consider what general relativity says about the effect of objects with mass on space itself. Mass causes the very fabric of space and time, united as 4-dimensional spacetime, to warp. Gravity arises from that curvature. The greater the mass, the more extreme the warping and thus the greater the force of gravity.
Here is the cool part: light usually travels in a straight line, but when the very fabric of spacetime is curved, it has to follow that path. So when light from a background object passes a foreground object, the light bends around it. The bigger the mass and the closer to that mass the light passes, the more its path is curved. That means light from the same source can reach our telescopes at different times. This causes an amplification of the background source.
This phenomenon of gravitational lensing has been used to great effect to study ancient galaxies that would usually be too distant and faint to see when they are gravitationally lensed by foreground galaxy clusters.
Obviously, planets have a heck of a lot less mass than clusters of galaxies, but they can still cause a slight gravitational lensing effect. That is micro-lensing, and it can be used to hunt planets.
Of the around 6,000 known exoplanets, only around 5 percent have thus far been discovered using microlensing. That is compared to around 75% found using the transit method TESS usually depends upon.
Gaia23bra b was first hinted at when it acted as a gravitational lens, passing between Earth and a background star, causing the ever-so-slight brightening of that star. The exciting thing about TESS successfully using microlensing is that this offers a complementary technique of exoplanet detection capable of detecting planets that the transit method might miss.
“With microlensing, we can find smaller planets with greater orbital distances, including worlds in the habitable zone of their star and even farther away,” team member Mallory Harris of the University of New Mexico said. “Microlensing events happen once, and they’re gone — they don’t repeat. I like to joke that we’ll probably find the first Earth analog with microlensing, and then wave at it as it goes by because we’ll never see it again.
And, if you will excuse the pun, the future is bright for microlensing. That is because it is one of the techniques that NASA’s next project, the Nancy Grace Roman Telescope, will use.
Roman will scour the very heart of the Milky Way where stars are tightly packed together, hunting microlensing events which should be common in such a dense stellar region. NASA scientists predict that this will lead to Roman discovering around 1,000 microlensing exoplanets on top of the estimated 100,000 transiting worlds it is predicted to detect.
“This is a bit like a preview of the microlensing NASA’s Nancy Grace Roman Space Telescope will do. The key to Roman’s microlensing survey is its dense time coverage targeting the galactic bulge,” team member Michael Fausnaugh of Texas Tech University said. “The TESS mission uniquely provides these rapid observations for stars in other parts of the galaxy, and pairing the two opens up prospects for understanding planet formation in a diverse population of stars.
“Since microlensing finds solar system-like planets, this offers a new chance to understand how planetary systems like our own vary in different regions of the galaxy.”
The team’s research was published on July 1 in The Astrophysical Journal Letters.
