Astronomers use new method to discover planet orbiting two stars

Astronomers using NASA’s Transiting Exoplanet Survey Satellite (TESS) telescope have used a new detection technique to discover an unusual planet that orbits two stars. The planet TIC 172900988b has two suns, making it a type of planet called a circumbinary, and it is the first of its kind to be detected using TESS observing just two transits.

Exoplanets, or planets outside our solar system, are usually too small and faint to be seen directly. But astronomers can infer their existence using a variety of techniques including transits. A transit is an event when an exoplanet passes between Earth and its host star, temporarily blocking out some of the star’s light. Astronomers look for these drops in light and use them to predict the presence of a planet.

Illustration of TIC 172900988b, a planet orbiting two stars.
Illustration of TIC 172900988b, a planet orbiting two stars. Dr. Pamela Gay / Planetary Science Institute

However, this is more difficult when a planet orbits two stars, as one of the researchers who discovered this planet, Planetary Science Institute Senior Scientist Nader Haghighipour, explained in a statement:

“Detecting circumbinary planets is much more complicated than detecting planets orbiting single stars,” Haghighiour said. “The most promising technique for detecting circumbinary planets is transit photometry, which measures drops in starlight caused by those planets whose orbits are oriented in space such that they periodically pass between their stars and the telescope. In this technique, the measurements of the decrease in the intensity of the light of a star is used to infer the existence of a planet.

“To determine the orbit of the planet, precisely, at least three transit events are required. This becomes complicated when a planet orbits a double-star system because transits will not happen with same interval over the same star. The planet may transit one star and then transit the other before transiting the first star again, and so on.”

A newly discovered planet was observed in the system TIC 172900988.
A newly discovered planet was observed in the system TIC 172900988. In TESS data, it passed in front of the primary star (right), and 5 days later (shown) passed in front of the second star (left). These stars are just over 30% larger than the Sun and differ very little in size. Dr. Pamela Gay / Planetary Science Institute

The problem is that detecting three transits can take a very long time — and TESS only looks at a given portion of the sky for 27 days, which is usually too little time to see three transits. But in the case of TIC 172900988b, a planet roughly the size of Jupiter, the team was able to detect it using just two transits — one transit of each of its host stars.

The authors are hopeful that this new technique means telescopes like TESS will be able to discover more circumbinary planets in the future.

The research is published in The Astronomical Journal.

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