A rare and valuable collection of transiting exoplanets on long-period orbits has been discovered hidden in data from NASA’s Transiting Exoplanet Survey Satellite (TESS) mission. Because exoplanet catalogues are dominated by short-period worlds close to their star, these longer-period candidates provide something different: specifically, insight into cooler planets.
“One of the great frontiers of exoplanet science is pushing out to these long periods that are comparable to those in the solar system,” says Faith Hawthorn, a final year PhD student at the University of Warwick who led the findings.
Longer-period exoplanets are less likely to transit than those closer in, and will do so more infrequently, making them harder to discover. Hawthorn and her team were able to work around this by taking advantage of the way that TESS surveys the sky. TESS spends a year (or “cycle”) observing one half of the celestial sphere, before moving onto the other half the following year. In cycles 1 and 3 it surveyed the southern sky, and in cycles 2 and 4, the northern sky. During each cycle the sky is split into sectors that TESS spends 27 days watching before moving onto the next sector. Consequently, TESS usually favours the detection of planets with orbital periods shorter than 10 days.
With the aid of an algorithm written by second author Sam Gill, also from Warwick, Hawthorn’s group searched the data from cycles 1 and 3, covering the southern celestial sky, for planets that transited twice, once in each cycle. They refer to these as “duotransits”, and they eschew the perceived wisdom of waiting to observe at least three transits to confirm the orbital period.
The algorithm initially picked out 2000 potential duotransits, and after vetting these by eye, Hawthorn’s team narrowed this down to 85. Twenty-five of these had already cropped up in analysis of the TESS data by other teams, but 60 were brand new. All appear to be gas giants, with the smallest being 2.67 times the radius of Earth, and they all require confirmation by radial velocity measurements to determine their mass.
Tantalizing transits
“Our technique exploits the way that TESS operates,” Hawthorn tells Physics World. “Other techniques, such as microlensing and astrometry, tend to contribute most of the long-period ones, but the important thing for us is that if you have a transiting planet you can also do transmission spectroscopy to look at their atmosphere.”
Transit spectroscopy involves measuring the imprint of a planetary atmosphere on a star’s light as that light is filtered through the atmosphere on its way to us. Molecules in a planet’s atmosphere leave dark absorption lines in a star’s spectrum, telling us the constituents of that atmosphere. Transit spectroscopy is now often performed on short-period worlds, but the opportunity to do it for longer-period worlds has not arisen often.
“If we truly want to understand how the atmospheres of exoplanets – and the exoplanets themselves – compared with those in the solar system, it’s these longer-period exoplanets that we need to study,” says Diana Dragomir, an astronomer at the University of New Mexico. Although Dragomir was not involved in Hawthorn’s study, she was part of a team that discovered two long-period duotransits in TESS data in 2023 and which also discovered hundreds of exoplanet candidates by employing an algorithm to detect single transits that had been missed by the conventional multi-transit techniques.
“I believe that there are still many single transits and duotransits in the TESS data that remain undiscovered,” Dragomir tells Physics World. “I do think that with improving algorithms we will find many of those in the coming years.”
Unusual candidates
Hawthorn’s candidate worlds have orbital periods that range between 20 and 700 days, although pinning down their precise period is impossible from just two transits. Most orbit standard F-, G- and K-type stars (our Sun is a G-type star, F-type stars are slightly hotter, K-type slightly cooler), but a few stand out as being different.
“It was nice that we did see a few unusual cases within what we found, but the caveat is they are just candidates for now,” says Hawthorn.
One system, designated TIC-221915858, has a hot A-type star (surface temperature 9200 °C, compared with the Sun’s 5500 °C) that would be the hottest star found by TESS to host a planet.
Another candidate is TOI-709, which involves a compact, evolved “hot subdwarf star” that has begun losing mass after its red giant phase and is on the way to transforming into a white dwarf. Another unidentified transit and a possible companion star muddy the waters.
“That’s a really weird one,” says Hawthorn. “It’s actually unlikely to be a planet, but we chose to keep it in the sample because it is so interesting and unusual. From our point of view looking at the data, it passed all our vetting tests. But there’s something really odd going on there.”
Similar to the solar system
If astronomers hope to find a broader range of planets like those in our solar system and not just those that orbit close to their stars, then embracing more unconventional techniques is key.
“Everyone kind of got used” to waiting for at least three transits, says Dragomir. “Perhaps, as a community, we should become more open to what properties are truly needed – or not – for a new exoplanet to be declared as such.”
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If the discoveries begin to mount up as advanced algorithms pick out more long-period planets hidden in the data, then astronomers will be able to perform statistical analyses to get a better sense of how common planetary system architectures like our solar system’s are.
“I would like to know how common they are relative to the closer-in planets,” says Dragomir. “In order to make this assessment we need as large a sample of longer-period planets as we can get.”
The discovery is reported in Monthly Notices of the Royal Astronomical Society.
