Astronomers are starting to get a general picture of these planets' formation, evolution, and composition. But the picture is much fuzzier for planets with longer orbital periods. Far-out worlds, with months- to years-long orbits, are more difficult to detect, and their properties have therefore been trickier to discern.
Now, the list of long-period planets has gained two entries. Astronomers at MIT, the University of New Mexico, and elsewhere have discovered a rare system containing two long-period planets orbiting TOI-4600, a nearby star that is 815 light years from Earth.
The team discovered that the star hosts an inner planet with an orbit of 82 days, similar to that of Mercury, while a second outer planet circles every 482 days, placing it somewhere between the orbits of Earth and Mars.
The discovery was made using data from NASA's Transiting Exoplanet Survey Satellite, or TESS - an MIT-led mission that monitors the nearest stars for signs of exoplanets. The new, farther planet has the longest period that TESS has detected to date. It is also one of the coldest, at about -117 degrees Fahrenheit, while the inner planet is a more temperate 170 degrees Fahrenheit.
Both planets are likely gas giants, similar to Jupiter and Saturn, though the composition of the inner planet may be more of a mix of gas and ice. The two planets bridge the gap between "hot Jupiters" - the toasty, short-orbit planets that make up the majority of exoplanet discoveries - and the much colder, longer-period gas giants in our solar system.
"These longer-period systems are a comparatively unexplored range," says team member Katharine Hesse, a technical staff member at MIT's Kavli Institute for Astrophysics and Space Research. "As we're trying to see where our solar system falls in comparison to the other systems we've found out there, we really need these more edge-case examples to better understand that comparison. Because a lot of systems we have found don't look anything like our solar system."
Hesse and her colleagues, including lead author Ismael Mireles, a graduate student at the University of New Mexico (UNM), have published their results in Astrophysical Journal Letters.
TESS monitors the nearest stars for signs of exoplanets by pointing at a patch of the sky and continuously measuring the brightness of stars in that sector for 30 days, before swiveling to the next patch. Scientists use "pipelines," or algorithmic searches, to comb through the measurements for dips in brightness that could have been caused by a planet passing in front of its star.
In 2020, one pipeline picked up a possible transit from a star in the northern sky, close to the constellation Draco. The star was categorized as TOI-4600 (a TESS Object of Interest). The initial transit was studied in detail by the TESS Single Transit Planet Candidate Working Group, a team of scientists at MIT, UNM, and elsewhere who look for signs of longer-period planets in single-transit events.
"For missions like TESS, where it only looks at each region of the sky for 30 days, you really need to stack up the number of observations to be able to get enough data to find planets with orbits longer than a month," Hesse notes.
The group looked for the star in other sectors of TESS data and eventually identified three more transits, similar to the first. From these four events, the scientists were able to determine that the source was a planet - TOI-4600b - with a relatively long 82-day orbit. The team also picked up a fifth transit, though it was out of sync with the other signals. They wondered: Could the transit be from another star temporarily eclipsing the first? Or could it be a second orbiting planet?
Giants in the sky
In 2021, when Mireles joined the group, he took up where the team left off, looking for more observations from TESS that would explain the last, puzzling transit.
"With each sector of data that came down, I would look to see if there was a second transit, and in the first five sectors, there wasn't," Mireles recalls. "Then, in July of last year, we saw something."
Actually, they saw two things: one transit that appeared in the same 82-day cycle, which further confirmed the existence of a long-orbiting planet; and a second transit, which was detected 964 days after the previous, out-of-sync transit. These last two transits were similar in depth, or the amount of light that was dimmed, suggesting that both were produced by a single object that was orbiting the star, either every 964 days, or every 482 days. After all, the team reasoned, TESS simply could have not been looking in the star's direction to catch the planet crossing at the 482-day mark. The team used a model to simulate what a planet would look like with both orbital periods, and concluded that the 482-day orbit was more likely.
To further confirm they had identified two long-period planets, the researchers focused in on the star using multiple ground-based telescopes. These observations helped the team rule out false-positive scenarios, such as a second star eclipsing the main star. In the end, they concluded that the star indeed hosts two long-period planets: TOI-4600b, a warm, Jupiter-like giant; and TOI-4600c, a frosty, icier giant, and the longest-period planet detected by TESS to date.
"It's relatively rare that we see two giant planets in a system," Hesse offers. "We're used to seeing hot Jupiters that are close in to their stars, and we usually don't find companions to them, let alone giant companions. This system is a more unique configuration."
The distance between the two planets, which is about the same as the space between Mercury and Mars, implies there could be other planets in the system.
"We want to see if there's evidence for more planets," Mireles says. "There's definitely a lot of room for potential planets, either closer in, or further out. And we show that TESS is capable of finding both warm and cold Jupiters."
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