by NASA’s Transiting Exoplanet Survey Satellite (TESS) may have found its first free-floating, or “orphan,” planet. That’s a planet wandering the starless cosmos, alone.
The promising discovery shows that TESS can use a phenomenon first proposed by Albert Einstein more than 100 years ago to spot so-called rogue planets.
Despite the fact that we know most about the planets orbiting a parent star (or star) having discovered more than 5,000 exoplanets that exist in such an arrangement, a large number of free-floating rogue planets are thought to exist The Milky Way. , too.
In fact, there may be as many as a quadrillion (10s and 14 zeros) in our galaxy that have had rogue planets thrown out of their home systems by gravitational interactions with other planets or bygone stars. That means these free-floating worlds could outnumber the stars across the Milky Way. Therefore, it is very likely that such an orphan could be detected by TESS, launched in 2018.
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“We found the first signal in the TESS data that is consistent with what would be expected from microlining at a free-floating planet,” said team co-leader Michelle Kunimoto, a postdoctoral fellow specializing in exoplanet detection at the Massachusetts Institute of Technology (MIT ). , said Space.com.
“This was just the first sector we searched through out of the 75 observed by TESS, with each sector corresponding to about 27 days of TESS observations,” Kunimoto continued. “It was surprising to find something so early – but very exciting.”
If this signal were to indicate a rogue exoplanet, the team tells Space.com, it’s likely to be a planet with a mass several times that of Earth no less than 6,500 light-years away.
A little “rogue hunting” help from Einstein
Most of the exoplanets detected so far have been characterized by the effect they have on their parent star. This could be a “settling” in the star’s motion caused by the gravitational tug of an orbiting minor planet, or a drop in light that occurs as an orbiting planet crosses, or “transits,” the face of its star.
Without a parent star, however, neither of these methods apply. That’s why it’s so hard to detect rogue planets.
“Rogue planets are dark, as you’d expect, and don’t orbit any stars, which means that the usual techniques for detecting exoplanets don’t work,” Kunimoto said.
Fortunately, Einstein’s 1915 theory of gravitation, better known as general relativity, predicts a phenomenon that can be used to find these free-floating exoplanets.
Einstein suggests that objects with mass curve the fabric of space and time, or spacetime, with gravity resulting from this curvature. When light passes one of those curved spots in space-time, its path is bent. This means that light from a background source, say a star or a galaxy, can take different paths around the intermediate “lensing” object, arriving at an observer’s vision at different times.
This phenomenon is called “gravitational lensing”, and results in the location of the background source shifting from the viewer’s perspective, or appearing in multiple locations in the same image.
Rogue planets have a very small mass, so the lensing effect is weak and thus called “microlensing.” However, it can brighten a background source visible to astronomers, indicating the presence of a rogue planet.
“Microlensing is the best – and usually only – option for finding these dark, remote objects since it only depends on the mass of a planet through its gravitational field,” Kunimoto said.
Forget the “T” in TESS
As the “T” for transit in TESS suggests, this space telescope might not immediately seem like the right instrument for hunting rogue planets.
“TESS is designed to search for transiting planets that are closely bound to their host stars,” explained Kunimoto. “Transitions and ‘dimmings’ of the star are caused by a planet passing in front of it, as you may have seen in the recent eclipse.”
However, as mentioned above, gravitational lensing can cause a background star to brighten as a lensing object passes between that star and Earth. Kunimoto explained that because TESS is sensitive to tiny changes in starlight, it can also detect these brightening episodes, a characteristic feature of microlensing caused by free-floating planetary debris.
But, given this, you might wonder: Why is this the first potential rogue exoplanet among the 6,000 or so other exoplanet candidates (400 or so confirmed) that TESS has seen since 2018?
Well, it turns out that no one was really looking until now.
“TESS is surprisingly well-suited to finding rogue planets through microlensing, but it turns out that these kinds of signals haven’t really been explored in TESS data,” Kunimoto said. “Our approach to looking for unbound planets with microlensing and the resulting TESS planetary microlensing candidate were a first for TESS.
“Since TESS data has not been used to search for short-period microlensing events before, no past exoplanet searches have been sensitive to seeing these signals.”
Unfortunately, however, like many other detected exoplanet candidates, this discovery still needs to be confirmed.
“It’s important to say that we can’t confirm right now that this is a planet,” Kunimoto said. “The fact that microlensing events do not repeat themselves means that it is difficult to identify the nature of any particular signal. Therefore, we are cautious about the origin of this event, naming it as a rogue planet ‘candidate’ because it come with the sign you” expect such a life.”
She added that as the team explores more TESS data and makes follow-up observations, the truth about the signal will slowly become clearer.
However, the tentative nature of these results certainly did not dampen the team’s enthusiasm or excitement.
“I’m definitely excited for ten,” William DeRocco, co-leader of the team and researcher at the University of California Santa Cruz, told Space.com. “I’m used to looking for dark matter, where the chances of seeing anything are very low, so the potential of something like a rogue world disappearing into the darkness of interstellar space to discover incredible.”
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The authors of this research believe that the future is bright when it comes to the prospect of TESS finding more rogue planets.
“This is a proof of principle that TESS can find these signals, and now we have to start diving deeper to find more and understand what they might mean,” said Kunimoto concluded. “We’ve searched through less than 1% of the TESS Data; with 99% to go, we have plenty of new opportunities for exciting discoveries along the way!
The team’s research has been submitted for publication in the journal Monthly Notices of the Royal Astronomical Society. It currently appears as a peer-reviewed paper on the arXiv repository site.
