Get to know the super-Earth that orbited the Milky Way 0:43

(CNN) --

Astronomers have discovered a "super-Earth," that is, a world larger than our planet, orbiting a star located about 137 light years away. A second planet, the size of Earth, could also orbit the same star.

The super-Earth exoplanet, known as TOI-715b, orbits a red dwarf star cooler and smaller than our Sun. Astronomers discovered the planet thanks to NASA's TESS (Transiting Exoplanet Survey Satellite) mission. A study about the discovery was published in January in the journal Monthly Notices of the Royal Astronomical Society.

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Researchers have determined that the planet, whose width is estimated to be one and a half times that of our planet, takes just over 19 Earth days to complete an orbit around its star. The planet is close enough to its star to be in the habitable zone, that is, at the distance from a star that provides a planet with the right temperature for liquid water to exist on its surface.

The habitable zone is usually calculated based on factors such as the size, temperature and mass of a star, as well as the reflectivity of a planet's surface. But there can be large margins of error associated with these factors, calling into question whether a planet actually resides in the habitable zone, says the study's lead author, Dr. Georgina Dransfield, a postdoctoral researcher at the School of Physics and Astronomy at the University of Birmingham, United Kingdom.

Astronomers believe that TOI-715b exists in a narrow, more optimal region around the star known as the conservative habitable zone, which is less likely to be affected by margins of error.

"This discovery is exciting as it is the first super-Earth detected by TESS within the conservative habitable zone," says Dransfield. "In addition, being relatively close, the system is suitable for future atmospheric research."


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TESS, the planet hunter

Since its launch in 2018, TESS has helped astronomers detect planets around relatively nearby stars, suitable for follow-up observations with ground-based and space-based observatories.

"This allows us to get a much clearer picture of the diversity of exoplanetary systems orbiting a wide range of stellar types," Dransfield said.

Telescopes can pick up dips in starlight that indicate the planet is passing in front of its star, and those dips in starlight are called transits. TOI-715b is close to its star and has a fast orbit, meaning the planet passes in front of, or transits, its star frequently. As a result, the exoplanet is an optimal candidate for future observations with the James Webb Space Telescope. The Webb telescope observes the universe in infrared light, invisible to the human eye, and can peer into the atmospheres of planets.

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As the planet transits the star, starlight filters through it, allowing Webb to look for signs of an atmosphere and even determine the planet's atmospheric composition. And knowing whether planets have atmospheres can reveal more about their ability to be potentially habitable for life.

"We want to know the mass of the planet with great precision to know if it is a true super-Earth or a member of the new category of ocean worlds," explains Dransfield, referring to moons with global oceans such as Europa, on Jupiter, or Enceladus, on Saturn. "This will allow us to really shape our follow-up investigations and learn more about the demographics of exoplanets as a whole."

To confirm the existence of the probable second Earth-sized planet, researchers need more successful observations of the planet's transits at different wavelengths of light, Dransfield said.

If the existence of an Earth-sized planet is confirmed, it will become the smallest TESS has yet found in a habitable zone.

The search for Earth-like planets

Red dwarf stars are the most common in our galaxy, and several of them have been found to host small rocky worlds, such as the recently discovered TRAPPIST system with its seven planets, located 40 light years away. Planets orbiting closer to these smaller, cooler stars could receive enough heat to be potentially habitable.

But a key question is whether these planets are also close enough to be hit by stellar flares and radiation, which could erode their atmospheres, evaporate water and limit their ability to be habitable for life.

TOI-715b's star has only shown a couple of flares in the past two years and is not considered active, making it an old star, Dransfield said.

In the future, astronomers hope to have the ability to search for planets around stars more like our sun, which will require the ability to block intense starlight to find faint Earth-sized planets.

Upcoming missions such as the European Space Agency's PLATO (PLAnetary Transits and Oscillations) will carry 26 cameras to study Earth-like planets in habitable zone orbits around Sun-like stars. The mission is scheduled to launch in 2026.

"So far, no telescope has been capable of this, but it should be possible within the next decade," Dransfield said, referring to PLATO. "It will be one of the most anticipated discoveries, as it will begin to show us how common Earth-like planets really are."