Most of the more than 5,000 planets known to exist outside our solar system orbit their stars at incredibly close distances. More than 80 percent of confirmed exoplanets have orbits shorter than 50 days, and these hot worlds are at least twice as close to their stars as Mercury is to our sun, and some are even closer.

A new discovery in distant worlds

Astronomers are starting to get a general picture of the formation, evolution, and composition of these planets. But for planets with longer orbital periods, the picture is much more blurry. Distant worlds, whose orbits take months or years, are more difficult to detect and therefore more difficult to detect.

Now the list of long-period planets is populated with two entries. Astronomers at MIT, the University of New Mexico and elsewhere have discovered a rare system that includes two long-period planets orbiting a nearby star, TOI-4600, 815 light-years from Earth.

The team discovered that the star contains an inner planet with an 82-day orbit similar to that of Mercury, while a second outer planet orbits every 482 days, placing it somewhere between the orbits of Earth and Mars.

Disclosure of TESS data

The valuable discovery was made using data from NASA’s Transiting Exoplanet Survey Satellite, or TESS, an MIT-led mission that monitors nearby stars for signs of exoplanets. The new, more distant planet has the longest period detected by TESS to date. It is also one of the coldest at about -117 degrees Fahrenheit, while the inner planet has a more moderate temperature of 170 degrees Fahrenheit.

Both planets are likely gas giants similar to Jupiter and Saturn, but the composition of the inner planet may be more of a mixture of gas and ice. The two planets bridge the gap between the “hot Jupiters” (frying, short-orbiting planets) that make up the majority of exoplanet discoveries, and the much cooler, longer-period gas giants in our solar system.

“These long-term systems are a relatively unexplored range,” says team member Kathryn Hesse, technical fellow at MIT’s Kavli Institute for Astrophysics and Space Studies. “We really need these better examples to better understand this comparison, as we’re trying to see where our solar system falls in comparison to the other systems we found out there. Because most of the systems we’ve discovered are nothing like our solar system.”

Hesse and colleagues, including lead author Ismael Mireles, a graduate student at the University of New Mexico (UNM), published their findings Aug. Astrophysics Journal Letters.

How does TESS work?

TESS monitors nearby stars for signs of exoplanets by pointing at a patch of sky and continuously measuring the brightness of stars in that sector for 30 days before moving on to the next patch. Scientists use “pipelines” or algorithmic searches to check for measurements of brightness dips that can be caused by a planet passing in front of its star.

In 2020, a pipeline recorded a possible transit from a star in the northern sky near the constellation Draco. The star was classified as TOI-4600 (TESS object of interest). The first transit has been studied in detail by the TESS Single Transition Planet Candidate Working Group, a team of scientists from MIT, UNM, and elsewhere who look for signs of long-period planets in single transit events.

“For missions like TESS where it only looks at every point in the sky for 30 days, you really need to accumulate enough observations to get enough data to find planets with orbits longer than the moon,” Hesse says.

discovery of giants

When Mireles joined the team in 2021, he picked up where the team left off, looking for more observations that would explain the latest mysterious transition from TESS.

“I looked to see if there was a second pass with every data sector that came in, but the first five sectors were not there,” Mireles recalls. “Then we saw something last July.”

They actually saw two things: a transit that occurred during the same 82-day cycle, further confirming the existence of a long-orbit planet; and the second pass detected 964 days after the previous asynchronous pass. The fact that these last two transits are similar in depth or amount of dim light suggests that they were both produced by the same object that orbits the star every 964 days or 482 days. Ultimately, the team figured that TESS couldn’t look in the direction of the star to detect the transit of the planet on day 482. The team used a model to simulate what a planet would look like in both orbital periods and concluded that an orbit of 482 days was more likely.

The researchers focused on the star with several ground-based telescopes to confirm they had identified two long-period planets. These observations helped the team rule out false positive scenarios, such as a second star outshining the main star. They eventually concluded that the star contains two long-period planets: TOI-4600b, a hot, Jupiter-like giant; and TOI-4600c, the cold, icy giant and the longest-period planet ever discovered by TESS.

“It’s relatively rare for us to see two giant planets in one system,” Hesse says. “We’re used to seeing hot Jupiters close to their stars, and we often don’t find any moons, let alone giant moons. This system is a more unique configuration.”

The fact that the distance between the two planets is close to the distance between Mercury and Mars means that there may be other planets in the system.

“We want to see if there is evidence of new planets,” Mireles says. “There’s definitely plenty of room for potential planets, both near and far. And we show that TESS can find both hot and cold Jupiters.” Source