Bad Astronomy | Exoplanet TOI-1452b is a super-Earth water world

A newly discovered exoplanet is indeed interesting: not only is it a relatively rare type of super-Earth, it may also be a water world, covered by deep oceans!

Maybe. But further observations, including observations on the JWST, can answer many questions.

The planet, called TOI-1452b, orbits the star TOI-1452. “TOI” refers to TESS’s Object of Interest; this in turn means a star observed by TESS (Transiting Exoplanet Survey Satellite), which may have a planet. Tess spent many days staring at hundreds of thousands of stars in search of exoplanets, alien worlds orbiting those stars.If a planet’s orbit is seen from the side, on each orbit it blocks a portion of the host star’s light as it passes directly in front of it – an event called transit. If we know the size of the star, then the size of the planet can be measured by the depth of the dip.

TOI-1452 is an M-type red dwarf, a star about 1/4 the mass and radius of the is much cooler than the sun, and a lot of Even fainter, emitting only 0.007 times the light of the Sun. It also has what appears to be a binary companion, another very similar red dwarf star about 15 billion kilometers away—three times the distance of Neptune from the sun.

According to TESS observations, exoplanet TOI-1452b has a diameter of about 1.67 times the diameter of Earth, making it a little over 21,000 kilometers wide [link to paper]. It’s fun right away. Planets roughly twice the diameter of Earth are called super-Earths, and they have the potential to resemble our own world. But if they get a little bigger, their atmospheres tend to be thicker and more Neptune-like, so those 2 to 4 times the diameter of Earth are called mini-Neptunes.

These are the most common planets orbiting other stars. However, the distribution has dropped between 1.5 and 2 times the diameter of Earth, and there are fewer planets in this range than you might expect. One reason could be that they started out more massive, but were so close to their star that the heat and light blew a lot of the atmosphere away, or that their cores were so hot after they formed that the internal heat expanded the atmosphere, and most It disappeared into space. There are other ways for planets to lose their atmospheres, but we need to find more in this “valley” to understand these mechanisms. Anytime you find something like TOI-1452b it would be helpful.

The planet orbits its star every 11.1 days, so its year is much shorter than ours, only a week and a half. Taking into account the star’s mass, this means that it orbits about 9 million kilometers from the star, or 1/16th The orbital distance between the Earth and the Sun. But remember, stars are dim; in the end, the planet gets about 1.8 times as much light from its star as we get from the sun. As a reasonable assumption, its temperature is between 25 and 50° C (about 80 – 130° F). Warmer than Earth, but not really.

This gets interesting when measuring quality. As the planet orbits the star, its gravity pulls on it, so while the planet forms a large circle, the star forms a small circle. The speed at which the star moves can be measured, which gives the planet’s mass: 4.8 ± 1.3 times the mass of Earth.

That’s a lot, but the planet is also bigger than Earth. It turns out that the planet’s average density is about the same as Earth, at 5.6 grams per cubic centimeter, or 5.6 times the density of water. This tells us, for example, that it’s not a gas giant like Saturn or Jupiter, whose average density is more like 1.

Using sophisticated models of planetary composition and structure, based in part on the abundance of elements in the host star, astronomers found that the planet likely had a small metallic core with a thick layer of rocky material. But the best fit to the data shows that Earth has a high proportion of water, a staggering 22% (plus or minus 21/13%) by mass. Earth’s water mass fraction is well below 1%, so if these models are successful, the planet is mostly water.

Given its temperature, that would be liquid water. So it could be a sea world. Also considering its temperature, muggy.

To be fair, another model suggests it might be more Earth-like, with far less water but hydrogen and helium.

There is one way to find out: observe it with JWST. During transits, some stars’ light passes through the atmosphere on its way to us, and different molecules in the air absorb different wavelengths of light, enabling them to be identified – this was recently done on planet WASP39b, showing its atmosphere in carbon dioxide. Water is a good absorber, especially in the infrared seen by JWST. The star is also bright in the infrared, so it’s not even that hard to observe. If you see – or don’t see – strong water absorption, that will quickly clear the character of the planet.

So don’t call Kevin Costner just yet.

I’ll add that red dwarfs are the most common type of star in the Milky Way, and they’re very good at making Earth-sized planets. There are seven TRAPPIST-1 systems! Planets around these dim bulb stars may outnumber all other planets in the sky, so understanding what they look like is one of the biggest goals of exoplanet science. Follow-up observations of TOI-1452b are expected to be a priority.

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