By combining data from two different telescopes, astronomers have discovered a rare exoplanet that faces destruction at the hands of tidal forces.
This artist’s impression shows a rocky ultra-short-period planet (USP). USPs such as TOI-6255 b have an orbital period of less than one Earth day. Credit: Pablo Carlos Budassi/Wikimedia Commons (CC BY-SA 4.0)
Sixty-six light years away, an Earth-sized exoplanet completes an orbit in less than six hours, revolving around its star at about one-hundredth the distance that Mercury orbits the Sun. This planet is on the verge of destruction.
Astronomer Fei Dai and his team at the University of Hawaii Institute for Astronomy combined data from the Transiting Exoplanet Survey Satellite (TESS) and the Keck Planet Finder (KPF) to officially verify the world, called TOI-6255 b, as an exoplanet. The discovery enables astronomers to observe two rare planetary phenomena: an ultra-short-lived planet that is nearing the end of its life. The study, published in August 2024 astronomical journalOne of the initial science results of the KPF and also demonstrates the research potential of the instrument.
pulled by the tide
Tidal forces arise when the gravitational pull from an external source varies due to the shape of an object. Just as Earth’s gravity pulls our feet stronger than our head, a star’s gravity pulls stronger on the near side of a planet than on the far side. But celestial bodies experience these forces on a much larger scale than we do. If a planet orbits too close to its star, the star’s tidal forces will overcome the planet’s structural integrity, tearing it apart completely. The limit where this happens is called the Roche limit.
Although TOI-6255 b currently orbits outside its star’s Roche limit, the planet still orbits its star so closely that the star’s tidal forces have stretched it like putty, molding it from a sphere into an egg. And complex interactions between the planet and its star can – and probably will – cause its orbit to decay. The researchers calculated that TOI-6255 b could exceed the star’s Roche limit in a few hundred million years – just the blink of an eye on a cosmic scale.
James Fuller, an astrophysicist at Caltech who was unaffiliated with the discovery, says the change could happen even sooner. When planets orbit close to their star for very short periods of time, the planets may lose their orbital energy and move closer due to the gravitational interaction between them. The exact pattern of this energy dissipation is highly dependent on the composition of the star and how much the star is distorted by the orbiting planet. Fuller explains that Dai and his team are “simply selecting a specific value [for that influence in TOI-6255 b’s case]. But that option could be worth 1,000 times less. “We don’t necessarily know.” Fuller points out that calculating this value from first principles is highly complex and notes that, given the information that Dai’s team had about the star, they made a best effort estimate and chose a reasonable value within the generally accepted range.
a unique opportunity
No matter how long it takes for the planet’s orbit to decay, TOI-6255 b presents a unique opportunity for astronomers. Dai’s team member and KPF instrument scientist Sam Halverson says most confirmed exoplanets orbit far enough beyond their star’s Roche limit that they pose no imminent threat of destruction. Although previous research has identified evidence of planets destroyed after crossing the Roche limit, TOI-6255 b is one of the few exoplanets found this close: a “convenient data point,” says Halverson. This presents a perfect opportunity for astronomers to see how these intense forces act on the planet’s surface as it orbits closer to its star.
TOI-6255 b’s impending demise isn’t its only unique feature. It is also a rare type of planet: an ultra-short-lived planet (USP). USPs orbit their stars in less than one Earth day. Earth’s orbit is 365 days long; TOI-6255b’s orbit is only 5.7 Earth hours. Of the more than 5,000 confirmed exoplanets, fewer than 150 USPs have been identified, and Fuller calls TOI-6255 b one of the most extreme examples of a USP discovered to date. Halverson agrees: “It’s actually a very interesting laboratory to investigate something that we don’t have [our] Solar system.”
With so few USPs identified, any new discoveries will help astronomers understand these planets as populations as well as individuals. Each USP provides new data about this class of planets, and the more we find, the better astronomers can piece together how they form, why there are none in our solar system, and how rare they really are in the universe.
combination of forces
But before astronomers can truly understand USPs, they must first be able to reliably detect and describe them. TESS watches exoplanets transit their star. It watches the star’s brightness decline as the planet passes in front of the star, collecting data including the planet’s radius and orbital period. KPF uses the radial velocity method to measure how much a star wobbles due to the gravitational pull of its orbiting planet. Measuring this wobble by studying the star’s slight motion toward and away from us reveals how much the planet’s gravity affects the star, thereby determining the exoplanet’s mass. Dai’s study demonstrates how combining TESS and KPF data enables astronomers to determine the density of exoplanets (calculating density requires both mass and size), which can then provide clues to their composition.
This combined approach allows astronomers to use TESS to target specific planets for KPF observations, resulting in more efficient and cheaper research. “Thankfully, because of missions like TESS, which have actually told us, ‘Here are your exact planetary periods and here are where to look,’ that makes it easier because we’re not searching blind,” Halverson says. And because this combined approach also enables astronomers to collect the information needed to outline the planet’s composition, astronomers will be better able to characterize rare exoplanets like USP than if they were using only KPF or TESS data.
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Tidal forces shape our solar system and our world every day. They pull Earth’s oceans into tides, pull and squeeze Jupiter’s moon Io and drive its volcanoes, and possibly even tear Saturn’s ancient satellites into pieces to form its rings. Light years away, these forces are reshaping a planet. Perhaps, over the last few hundred million years of its existence, TOI-6255 b will teach us about rare planets and the destructive tidal forces they endure – before its sun destroys it.
