Light curves recorded by the TESS satellite are now being examined by volunteers looking for possible black holes. The Black Hole Hunters project does not ask them to observe one directly: instead, they look for patterns in the brightness of stars that could reveal the gravitational influence of an invisible companion. A signal that catches their eye is still only a lead to be checked, not a confirmed discovery.
Why look for an object that emits no light?
A black hole is difficult to detect from its own light: its gravity is so intense that nothing, not even light, can escape once it has crossed a boundary called the event horizon. But an invisible object can leave measurable effects on its surroundings. In some systems, the light from a companion star can be distorted or amplified when the black hole passes in front of it, as seen from Earth. This gravitational lensing effect is sometimes called ‘self-lensing’ in a binary system: it is the light from the system’s star that is affected.
This principle should not be confused with conventional gravitational microlensing, in which a foreground object temporarily amplifies the light from a background star, often without forming a pair with it. In the case sought by Black Hole Hunters, researchers are interested in systems where a black hole could be bound to a star and produce a particular variation in its brightness. Such a variation may be consistent with the hypothesis of a black hole, but other explanations must be ruled out before reaching a conclusion.
TESS measures variations in light
Launched by NASA to search for exoplanets, TESS — the Transiting Exoplanet Survey Satellite — monitors the brightness of stars. Among other things, it looks for regular dips in light that can occur when a planet passes in front of its star. These repeated measurements form light curves: graphs showing how an object’s brightness changes over time.
The mission therefore collects a volume of data that can be used to investigate questions beyond the search for planets. A light curve does not directly show the shape of a black hole. It represents a measurement of brightness, which scientists then interpret by taking into account the period, shape and repetition of variations, as well as possible alternative explanations.
NASA says that TESS completed its two-year primary mission and is continuing its observations under extended missions. The mission page notably features a view of the sky published with confirmed or candidate exoplanets identified up to the end of the second extended mission, in September 2025. Using its data to search for black holes is another way of making use of the satellite’s measurements, not a change in what it observes: TESS continues to measure the light from stars.
What the volunteers actually see
According to the University of Southampton, the project team first used an artificial intelligence system to sift through millions of possible locations in the Milky Way and select data for examination. The researchers turned some of this data into graphs and animations for participants. They can flag patterns that seem interesting to them, helping the team decide which leads merit further analysis.
The task, then, is visual pattern-spotting, not independently identifying a black hole. An unusual pattern may be a candidate; on its own, it does not prove that the object being sought is present. Researchers must revisit the measurements, test the hypotheses and, if necessary, seek additional observations. Southampton’s press release describes the volunteers as helping to target future telescope observations, rather than as making an immediate confirmation.
This distinction matters: an animation or graph is a representation of processed data, not a photograph of a black hole. The colours or shapes used to make the variations easier to see are not, in themselves, what a human eye would see in the sky. The scientific result depends on the underlying measurement and its interpretation, not on how spectacular a visualisation looks.
A public-participation search, with a specific role
The University of Southampton explains that the number of known black holes in the galaxy remains far below the number researchers believe may exist. It says its team has narrowed the vast body of data down to cases that volunteers can examine. These are the estimates and account given by the project team; they do not mean that thousands of new objects have already been found.
Here, the value of citizen participation lies in a concrete step: examining a large number of graphs to flag patterns that might escape a fully automated screening process or require a human eye. The computer helps reduce the volume to be processed; participants draw attention to cases; researchers then assess the candidates. Each stage has its limits and does not replace the others.
Black Hole Hunters thus illustrates one way of doing science with data already collected by a space mission. Any eventual discovery will depend on what the curves can actually establish and on the checks that follow. For now, the call for volunteers is about looking for possible signs in the light from stars — not confirming that those signs really are those of a black hole.
References
- Scientists enlist armchair astronomers to find thousands of black holes hiding in our galaxy — University of Southampton
- TESS (Transiting Exoplanet Survey Satellite) — NASA Science
Photograph: NASA / Kim Shiflett — Wikimedia Commons — Public domain.
