In a very distant planetary system, about 1,113 light years from Earth, intuition asks us for a very reasonable thing: if a planet is almost the size of Jupiter, it should also resemble Jupiter in its mass. NASA’s TESS mission just showed that the universe doesn’t always play by those rules. From your data, scientists have identified two giant worlds around the star TOI-791 that seem made to break that expectation: they take up a lot of space, but concentrate very little matter.
The discovery has its own names: TOI-791 by TOI-791 c. They are two “super-puff” planets, a term used to describe giant worlds with extremely low densities, comparable in this case to that of cotton candy. Scientists calculate that they are the most “bloated” planets found so far, a striking label but supported by a very specific comparison: their size is close to that of Jupiter, while their mass represents only a small fraction of that of the largest planet in the Solar System.
The key piece of this story is TESSNASA’s Transiting Exoplanet Survey Satellite. We are not talking about a telescope designed to obtain direct images of these worlds, but rather a space observatory prepared to monitor large areas of the sky looking for indirect signalss. According to technical information, its payload is concentrated in a single instrument: a set of four wide-field optical cameras. These cameras work together with their covers, mount, sun shield and data management unit to track stars for long periods.
Two giant planets that weigh almost nothing
The important thing is that TESS did not see those planets the way we see an image of Jupiter or Saturn. What it detected were repeated small drops in the brightness of TOI-791, the Sun-like star that hosts this system. This pattern appears when, from our perspective, a planet passes in front of its star and blocks a minimal part of its light. From these transits, and how they repeat over time, scientists can reconstruct the presence of worlds that are too far away to be shown as a conventional photograph.
There’s an understandable trap here: we see the NASA illustration and our brain fills in the scene as if we were looking at a photo. But that is not what has happened. The agency clarifies that no direct image of TOI-791 by TOI-791 cand that its appearance in the visual pieces is an artistic interpretation. The image serves to bring us closer to the discovery and compare it with known planets, but it is not the observation itself: the observation is in the signals measured by TESS when these worlds pass in front of their star.
The TESS spacecraft and its payload, prepared before launch
The rarity appears very clearly when the figures come in. TOI-791 b is almost the same size as Jupiter, but contains only 3.0% of its mass. TOI-791 c goes even one step further: it is larger than Jupiter, although it barely reaches 5.9% of its mass. That combination is what makes these worlds so strange. We are not dealing with small planets with little matter, but with giants that take up a lot of space and yet concentrate a surprisingly low amount of mass.
There is also a question of patience. TOI-791 b takes 139 days to complete one revolution around its star, and TOI-791 c needs 232 days. For a telescope that searches for planets through transits, that means waiting a long time to see the same signal repeat and confirm that we are not facing a coincidence. The accumulation of data was decisive here: from its high orbit around the Earth, TESS gathered 1,122 days of observations of this system over seven years.
The image compares the size of the two “super-puff” planets with some worlds in our Solar System
To arrive at their masses, scientists took advantage of a very useful detail: these two planets do not move as if the other did not exist. TOI-791 b and TOI-791 c follow an orbital pattern that causes them to be gravitationally attracted to each other. That push and pull slightly changes the timing of its transits across the star from our perspective. By measuring these small temporal variations, the team was able to estimate how much mass each planet contains and confirm their status as low-density “super-puff” planets.
The confusion comes not only from the fact that they are huge worlds with very little mass, but also from the fact that they fit poorly with what was expected to be found. Jon Jenkins of NASA Ames sums it up this way: “They represent a puzzle “We must solve how giant planets like Jupiter and super-puffs form.” George Dransfield, lead author of the study at the University of Oxford, also emphasizes that their extremely low densities make them fascinating targets for studying the evolution of planetary systems. The metaphor, in fact, was the gateway to the problem.
What comes next is trying to read those worlds in more detail. NASA notes that scientists want to study the chemical composition of their atmospheres, how their rotation can affect their shape and to what extent the star’s inclination fits with the orbits of the planets. It also remains to be understood how they moved within the system during its development, whether their orbits were shaped by interactions with other planets and, ultimately, how such low-density worlds can form. Cotton candy was the image; The challenge is to explain the recipe.
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