Exoplanets

TESS has Found A Second Earth-Sized World in This System. Exoplanet Science is Maturing

For planet-hunters, finding an Earth-sized exoplanet must be special. NASA estimates there are about 100 billion planets in the Milky Way, but the large majority of the 5,000+ exoplanets we’ve found are extremely inhospitable. So finding one that’s similar to ours is kind of comforting.

In this case, it’s even more interesting because it’s the second Earth-sized planet orbiting the same star.

The newly-found planet is named TOI 700-e. The star, TOI 700, is a red dwarf just over 100 light-years away in the Dorado constellation. TESS has observed the star before and found three planets orbiting the star, making TOI 700-e the fourth.

TOI 700-e is in the star’s habitable zone, a region where liquid water could exist on the planet’s surface given the right atmospheric conditions. It’s about 95% of Earth’s size, and it’s probably also rocky. Its sibling, TOI 700-d, is also a rocky planet about 1.1 times the size of Earth that orbits in the star’s habitable zone. The fact that there are two rocky planets about the same size as Earth, both in the habitable zone, puts the TOI 700 system in a rare category.

TOI 700-e, illustrated here at left, is around 95% Earth’s size and likely rocky. It’s on a 28-day orbit around its star. At right, TESS observations of the planet’s transit. Credit: NASA/JPL-Caltech/Robert Hurt/NASA’s Goddard Space Flight Center

“This is one of only a few systems with multiple, small, habitable-zone planets that we know of,” said Emily Gilbert, a postdoctoral fellow at NASA’s Jet Propulsion Laboratory in Southern California who led the work. “That makes the TOI 700 system an exciting prospect for additional follow-up. Planet e is about 10% smaller than planet d, so the system also shows how additional TESS observations help us find smaller and smaller worlds.”

Gilbert and her colleagues presented their results at the 241st meeting of the American Astronomical Society. The Astrophysical Journal Letters has accepted their paper for publication: “A Second Earth-Sized Planet in the Habitable Zone of the M Dwarf, TOI-700.”

TESS found the first three planets a couple of years ago, and the discovery made a splash. TOI 700-d was TESS’s first Earth-sized exoplanet in a star’s habitable zone, and finding it was a milestone for the mission. Gilbert was also the lead author of the paper announcing those findings. TOI 700-e was more elusive than its siblings, and it remained a candidate until TESS observed more of its transits.

The term habitable zone is used to denote planets the right distance from their star that could allow liquid water on their surfaces. But as we move deeper into the age of exoplanet discovery, scientists are using more precise terms. The optimistic habitable zone (OHZ) is a region where liquid surface water might’ve existed at some point in the planet’s history. The conservative habitable zone (CHZ) is a more tightly-constrained region than the optimistic habitable zone. It’s a region where scientists hypothesize that liquid surface water could’ve existed for most of a planet’s history. The optimistic habitable zone extends to either side of the conservative habitable zone.

The OHZ is based on estimates of when Venus and Mars may have had liquid surface water, and the CHZ boundaries are defined by the runaway and maximum greenhouse limits.

TOI 700-e is in the optimistic habitable zone, where it’s more likely that surface water and an Earth-like atmosphere could exist. There’s no indication that 700-e has an Earth-like atmosphere; the term optimistic habitable zone just says it’s more likely than the conservative habitable zone.

This image shows the Conservative and Optimistic Habitable Zones around TOI 700. Image Credits: NASA

All of the planets are likely tidally locked to their star, and their orbital periods are short relative to our Solar System’s planets. They range from 10 days for the innermost planet 700-b to just over 37 days for the newly-discovered 700-e. Planets b, d, and e are likely rocky, while planet c is likely more similar to Neptune.

This discovery shows how exoplanet science keeps advancing. When NASA launched its Kepler planet-hunting spacecraft in 2009, we knew of only a handful of exoplanets. When Kepler ceased operations in 2018, it had detected just over 2600 exoplanets. Now we know of more than 5200 exoplanets, and the more we discover, the more we’re building a representative sample.

This is a screenshot of NASA’s interactive timeline of exoplanet discoveries. Click on the image to visit it. Image Credit: NASA

“The field of exoplanets has come great lengths over the past several decades,” the authors write in their paper. “In the early days of exoplanet science, the focus was largely on individual planet detections, showing that these worlds even exist and that astronomers are capable of detecting them.”

Most of the planets found so far are nothing like Earth, and many are nothing like the other planets in our Solar System. And in the early days of exoplanet discovery, we didn’t have the tools to reliably detect small, Earth-like planets. Our detection methods had a built-in sampling bias: we found lots of gas giants.

But these results show how things are changing, and the change began with Kepler. “In the 2000s and 2010s, the introduction of new technology and facilities pushed the field closer to discovering Solar System analogs,” the authors write. “The Kepler mission, in particular, observed with high-enough sensitivity to routinely detect Earth-sized planets on temperate orbits around M dwarf stars, leading to the discovery of the first Earth-sized HZ planet (Kepler 186-f.)”

Kepler 186f was our first Earth-sized habitable zone planet. It’s about 580 light-years away. Image Credit: NASA Ames/SETI Institute/JPL-CalTech

Now we know that multi-planet systems like ours are common. We also know that Earth-size planets in habitable zones are more common than we once thought, something we could only wonder about wistfully ten years ago.

But even with all of the advances, we’re still a little hamstrung. The transit method of discovery limits our search to solar systems oriented edge-on to ours. And most of Kepler’s stars were too faint to allow further study by ground-based telescopes. “Without knowledge of their bulk compositions and atmospheres, it was impossible to discern whether the habitable-zone Earth-sized planets Kepler found in spades were truly Earth-like,” Gilbert and her co-authors write.

But as exoplanet science matures, those obstacles don’t loom as large.

This image shows the exoplanet HIP 65426 b in different bands of infrared light, as seen from the James Webb Space Telescope. This is the first exoplanet imaged by JWST. Credit: NASA/ESA/CSA, A Carter (UCSC), the ERS 1386 team, and A. Pagan (STScI).

The James Webb Space Telescope is helping launch the next phase of exoplanet science. Its powerful instruments are designed to study exoplanet atmospheres and use spectroscopy to determine their compositions. It’s already done so with one planet, WASP 39b, and discovered that the hot Jupiter’s atmosphere contains sodium, potassium, carbon dioxide, carbon monoxide and water vapour. Perhaps most significantly, it also contains sulphur dioxide, which indicates that photochemical reactions are taking place in the atmosphere.

It’s the first time scientists have found the molecule anywhere outside our Solar System. The detection is significant because it shows that the JWST can detect photochemistry and should be able to detect other photochemistry, like ozone, in the atmospheres of Earth-like planets.

One of the JWST’s objectives is to study exoplanet atmospheres, and it’ll teach us a lot more about them. But the JWST is busy and has a long list of targets. Who knows if the planets in the TOI system will be on its list?

In their paper, the authors explain TOI 700-e’s likely properties.

“With a radius of 0.953 Earth radii, we found that TOI-700-e is likely a rocky planet with a probability of 87%,” they write. They also give a mass estimate of 0.845 Earth masses, though they acknowledge the difficulty in determining that. The researchers also write that “… we calculated the timescale for tidal locking of TOI-700-e to be on order a few million years. Given the age of the system, it is likely that the planet is in a locked-in synchronous or pseudo-synchronous rotation.”

Determining habitability is impossible at our stage of exoplanet science. We can rule out habitability for the large majority of planets we find. Most of them aren’t in habitable zones, many are hot Jupiters, and some are bizarre planets with exotic atmospheres that might contain things like iron vapour. But that same certainty doesn’t extend to HZ planets. HZ planets are where the action is, and scientists simply can’t see those planets well enough to make firm conclusions. All scientists can say is that a planet might be potentially habitable and then explain their detailed findings.

That’s what Gilbert and her colleagues have done with TOI-700-e. “As shown in Figure 4, planet d resides within the CHZ, and the orbit of planet e is within the OHZ,” they write.

This figure (Fig. 4) from the paper shows that the outermost planet d is in the conservative habitable zone (dark green) and that the newly-discovered planet e (pink) is in the optimistic habitable zone (light green.) The other two planets are in neither zone. Image Credit: Gilbert et al. 2023.

But something sets the TOI 700 planets apart from many planets in other HZs. Most of the HZ planets we find are orbiting M-dwarfs, also called red dwarfs. These types of stars are known to be very active and can emit powerful flares that could make life on their HZ planets unlikely. But TOI 700 is different; it seems to be much more quiescent.

“Most importantly, TOI-700 is relatively quiet and bright enough that we can now take the next steps to further characterize this system and learn more about its HZ planets. This lack of activity is also greatly beneficial in terms of planetary habitability,” the authors write. “We see no signs of flares in the TESS optical photometry, and TOI-700 is quiet when observed by HST in the UV, which is valuable in terms of planets’ abilities to retain their atmospheres over time.”

Artist’s impression of a flaring red dwarf star orbited by an exoplanet. Red dwarfs are known for powerful flaring that can make planets uninhabitable even if they’re in the star’s habitable zone. Credit: NASA, ESA, and G. Bacon (STScI)

These are intriguing hints, and since TOI is relatively quiet, it can make a good comparison with another well-known red dwarf system with multiple planets in its HZ. The TRAPPIST-1 system has four planets in its potentially habitable zone, but the TRAPPIST-1 star is rambunctious compared to TOI-700.

“A comparison between these planets may elucidate how the stellar environment affects planets’ ability to maintain their atmospheres and what their compositions are like,” the team writes. “The more systems we can study with multiple HZ planets, the better our understanding of these worlds will be.”

And the more exoplanet science will keep maturing.

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Evan Gough

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