A nearby super-Earth could be an aquatic world

A nearby super-Earth could be an aquatic world

A recently discovered nearby exoplanet proved interesting: not only is it a relatively rare kind of super-Earth, but it could also be an aquatic world, covered by a deep ocean!

Maybe. But further observations, including with JWST, could answer a lot of questions.

The planet is called TOI-1452b, orbiting the star TOI-1452. “TOI” means the TESS Object of Interest; which in turn means a star observed by TESS – the Transiting Exoplanet Survey Satellite – which may have a planet. TESS watches hundreds of thousands of stars over several days looking for exoplanets, alien worlds orbiting those stars. If a planet’s orbit is viewed edge-on, then once per orbit it blocks a fraction of the light from its host star as it passes directly in front of it – an event called a transit. If we know the size of the star, the size of the planet can be measured by the depth of the dip.

TOI-1452 is an M-type red dwarf star about ¼ the mass and radius of the Sun. It’s much cooler than the sun and a lot fainter, emitting just 0.007 times more light than the Sun. It also has what appears to have a binary companion, another very similar red dwarf about 15 billion kilometers away – three times as far as Neptune’s distance from the Sun.

According to TESS observations, exoplanet TOI-1452b is about 1.67 times the diameter of Earth, so just over 21,000 km wide [link to paper]. It’s immediately interesting. Planets up to about twice the diameter of Earth are called super-Earths and have a chance of being similar to our own world. If they grow a little larger, however, they tend to have much thicker, more Neptune-like atmospheres, so those 2 to 4 times the diameter of Earth are called mini-Neptunes.

These are the most common types of planets orbiting other stars. However, there is a dip in the distribution between about 1.5 and 2 times the diameter of Earth, with fewer planets in this range than you would expect. One reason could be that they started with more mass but got so close to their stars that the heat and light turned up much of the atmosphere, or that their cores are so hot after forming that the heat inner atmosphere swells, and much of it is lost to space. A planet can lose its atmosphere in other ways, but we need to find a lot more of it in this “valley” to understand the mechanisms. Whenever a TOI-1452b is found, it helps.

The planet orbits its star every 11.1 days, so its year is much shorter than ours, just a week and a half. Given the star’s mass, that means it orbits about 9 million km from the star, 1/16e Orbital distance from the Earth to the Sun. But remember, the star is weak; In the end, the planet receives about 1.8 times more light from its star than we receive from the Sun. For reasonable assumptions, it has a temperature between 25 and 50°C (about 80 – 130°F). Hotter than Earth, but not wildly.

This becomes interesting when mass is measured. As the planet orbits the star, its gravity pulls on it, so the star makes a small circle while the planet makes a large one. The speed of the star as it moves can be measured, giving the mass of the planet: 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’s, 5.6 grams per cubic centimeter, or 5.6 times the density of water. This tells us, for example, that it is not a gas giant like Saturn or Jupiter, which have an average density closer to 1.

Using sophisticated models of planetary compositions and structures based in part on the abundance of elements observed in the host star, astronomers have found that the planet is likely to have a small metallic core with a thick layer of material. rocky above. But the best fits to the data indicate a high fraction of water on the planet, a shocking 22% by mass (plus or minus 21/13%). Earth’s water fraction by mass is well below 1%, so if these patterns hold out, this planet is largely made up of water.

And given its temperature, it would be liquid water. It may therefore be an oceanic world. Also given its stifling temperature.

To be fair, another model indicated that it might look more like Earth, with a lot less water, but with an atmosphere of hydrogen and helium.

There’s a way to find out: watch it with JWST. During a transit, some of the light from the star will pass through the atmosphere on its way to us, and different molecules in the air will absorb different wavelengths of light there, allowing them to be identified – this has been done recently with the planet WASP39b, showing carbon dioxide in its atmosphere. Water is an excellent absorber, especially in the infrared where JWST sees. The star is also bright in the infrared, so it wouldn’t even be such a difficult sighting. Whether strong water uptake is observed – or not – would shed some light on the character of this planet fairly quickly.

So don’t call Kevin Costner yet.

I will add that red dwarfs are the most common types of stars in the galaxy and they are very good at creating Earth-sized planets. The TRAPPIST-1 system has seven! The planets around these dim bulb stars may outnumber any 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 will hopefully be a priority.

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