Scientists have discovered that a distant “hot Jupiter” planet has a planetary companion. The two planets are locked in a rhythmic dance around the star TOI-1408, which lies 455 light-years from Earth.
TOI-1408 b was first discovered last year by NASA’s Transiting Exoplanet Survey Satellite (TESS) spacecraft, and is a gas giant planet that is twice as wide as Jupiter and roughly the mass of the other gas giant planets in the solar system. It orbits its parent star at a distance of 5.3 million miles, or about 6 percent of the distance between Earth and the sun. That means TOI-1408 b completes its orbit in just 4.4 Earth days.
Because of its size and proximity to its star, TOI-1408 b is classified as a “hot Jupiter,” but its newly discovered companion is even closer to the system’s parent star. TOI-1408 c is only 3.3 million miles from its star, meaning it completes an orbit in 2.2 Earth days. However, this newly discovered planet is not a hot Jupiter.
With a diameter roughly twice that of our planet and a mass eight times that of Earth, TOI-1408 c is classified as a “super-Earth” or “sub-Neptune.” Unlike TOI-1408 b, which is mostly made of gas, TOI-1408 c is likely to be made of a mixture of gas and rock.
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“Both TOI-1408 b and c are incredibly close to their parent star compared to planets in our solar system,” lead author Judith Kurth of Lund University told Space.com. “Imagine our solar system, but instead Jupiter orbits the Sun about every four days, one-twentieth the period of Mercury.
“This is already very close to the star, and yet we have discovered another planet even closer to the star that interacts strongly with its larger neighbor, causing their orbits to wobble in ways we have never seen before.”
While it was surprising to find an exoplanet, or “exoplanet,” in the TOI-1408 system closer to its star than TOI-1408 b, finding a gas giant with its orbit so disturbed was even more bizarre, Court explained.
“Finding a hot Jupiter like TOI-1408 b with a turbulent orbit is really interesting because it’s unusual for such a large planet,” she added. “But finding a smaller planet that’s moving the larger planet around in its orbit was even more unusual.” “This discovery challenges some of our assumptions about how these planetary systems form, as we don’t usually find smaller, less massive planets in the orbit of a hot Jupiter.”
Rhythmically dancing worlds
But there’s something else that makes this system exceptional: its two planets dance a delicate rhythmic dance. Although the smaller planet TOI-1408c is tugging at its hot Jupiter dance partner, the TOI-1408 system appears to have a stable configuration that allows the two planets to coexist in close orbits.
“The system’s dynamics are finely tuned, probably due to its formation history. The planets’ orbits are roughly 2:1, meaning that for every two orbits of TOI-1408c, TOI-1408b orbits the star once,” Kurth said. “This may help stabilize their orbits despite their close proximity to each other and to the star.”
She added that the exact reason why the TOI-1408 system formed in this way is most likely the movement of planets through the disk of gas and dust that surrounded its star in its early stages, which is called the “protoplanetary disk” where the planets formed.
TESS has become an exceptional exoplanet hunter, discovering more than 7,000 potential exoplanets since its launch in 2018. TOI-1408 is two of the latest exoplanets.
NASA’s exoplanet hunter has been studying 200,000 of the brightest stars in the Sun’s vicinity, watching for planets as they transit, or “cross,” in front of their parent stars, causing a slight dip in starlight.
TOI-1408 is just such a star. With a mass 1.3 times that of the Sun and 1.5 times the width of our star, TOI-1408 is slightly hotter and brighter than the Sun. While TESS has discovered both TOI-1408 b and c, the smaller planet has been a bit harder to spot.
“TOI-1408 c was difficult to detect because its transit — when it passes in front of the star from our point of view — is much shallower, meaning it blocks less light from the star than TOI-1408 b,” Kurth said. “In addition, TOI-1408 c’s transits are affected by large variations in transit timing (TTVs), meaning its transit timing is not as predictable as TOI-1408 b.”
These stellar events are caused by the gravitational interaction between the two planets, which means more sophisticated analysis is required to detect TOI-1408 c, Kurth added.
“We discovered TOI-1408 c by carefully analyzing light curves from the TESS satellite for TTVs of the larger planet TOI-1408 b,” she continued. “These TTVs can be produced by gravitational interaction with another planet, yet the TESS team did not know of any other planet.”
To detect a transit signal that might come from another, smaller planet with a shorter orbital period than the known planet, the team used photodynamic modeling, which combines transit light curves and radial velocity data to account for complex interactions between planets.
The team has not finished studying the TOI-1408 system yet, as evidence suggests it may contain at least three worlds. However, if there are more planets in the system, they are unlikely to be within the orbit of TOI-1408 c.
“There are hints that there might be a third, more distant planet in the system, based on radial velocity data that show a likely trend for a long time. However, for planets closer than TOI-1408 c, we have not found any evidence of this yet,” Kurth said. “TOI-1408 c is already very close to the star, and it would be difficult for another planet to maintain a stable orbit inside it.”
“The TOI-1408 system is incredibly interesting, so we definitely plan to continue studying it,” Kurth concluded. “We want to better understand the potential third planet and explore the long-term stability of the system. However, we are also continuing to explore other planetary systems.”
“Every new discovery like TOI-1408 c helps improve our models and understanding of how planets form.”
The team’s research has been published on the paper repository. Arcef.
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