Exoplanet Evolution: From Mini-Neptune to Super-Earth

When the mini-Neptune planet lost its atmosphere, this type of planet evolved into a super-Earth rock planet.

This is what happened to the two mini-Neptune exoplanets that astronomers observed. The stellar radiation strips the atmospheres of both planets. As a result, the hot gases on the planet evaporate like boiling water vapor.

Neptune-mini and Earth-Super

In the Solar System we know the rock planets and the gas planets. In the ranks of the rocky planets are Mercury, Earth’s Venus, and Mars. Earth is the largest of the 4 planets. Meanwhile in the ranks of the gas planets there are Jupiter, Saturn, Uranus, and Neptune. Jupiter is the largest planet and Neptune is the smallest gas planet. Interestingly, rock planets are close to the Sun, while gas planets are actually far from the Sun.

And this is the standard for the types of planets expected to be found in other stars. In fact, since exoplanets were discovered, astronomers have been surprised. The planets found have a different type from those in the Solar System.

A gas giant planet, but its location is precisely near the star. Not as far from the stars as in the Solar System. Astronomers categorize this exoplanet as a hot Jupiter

In addition there are rock planets but much larger than Earth or gas planets similar to Neptune but smaller, denser, with large rocky cores and thick gas envelopes. These two types of planets are then categorized as super-Earth planets and mini-Neptune.

However, there is something interesting.

The super-Earth planets discovered to date are 1.6 times the diameter of Earth, while mini-Neptune is between 2 and 4 times the size of Earth. Only a few planets have been found with sizes between 1.6 – 2 times the size of Earth.

Why is there a gap between the size of Earth-super and Neptune-mini?

Gap Ukuran Exoplanet

To explain the gap in planet-sized discoveries between super-Earths and mini-Neptune, astronomers have developed several theories.

One of the explanations given is that the mini-Neptune planet evolved into a super-Earth planet.

When it formed, the mini-Neptune planet was in a cocoon of a primordial atmosphere dominated by hydrogen and helium. These hydrogen and helium gases are the remnants of the central star-forming gas in the system.

If the planet Neptune formed was small enough and close to the star, X-ray and UV (ultraviolet) radiation from the star would strip the planet’s atmosphere. This process continued for hundreds and millions of years until finally the thick gas envelope of the mini-Neptune planet left only a fairly large rock core with a thin atmosphere like other rock planets.

Mini-Neptune-type planets can’t stay in the gap for long because planets with thick cocoons of gas have strong enough gravity to attract more gas and reach larger sizes. Or even the gas cocoons are thinner so that they are easily swept away by the radiation of the stars and the planets are transformed into super-Earth planets.

Another scenario that can explain the gap is that small rocky planets have failed to accrete or capture gas from the start. And the planet Neptune-mini is a water planet that is not enveloped in gas.

The discoveries of astronomers actually support the idea of ​​​​transforming a mini-Neptune planet into a super-Earth planet.

Caught in the act

Observations with the telescope at the WM Keck Observatory at the top of Mauna Kea, Hawai’i caught more than one planet losing its atmosphere. Astronomers witnessed how the two planets were “evaporating” when heated by stellar radiation.

These two planets also do not orbit one star. This is the result of observations of two systems on two different stars. The first is the star TOI 560 which is 103 light years from Earth, and the star HD 63433 at a distance of 73 light years.

The first star, TOI 560 or HD 73583. We named the planet TOI 560.01 or HD 73583b. It is 2.8 times the diameter of Earth and orbits a star cooler than the Sun with an orbital period of 6.4 days. The distance to the star is quite close at 9 million km and of course at this close distance, the planet is also very hot.

Observations showed helium was released from the planet, moving at a speed of 20 km / sec. Interestingly, the gas that escapes is not moving away from the star after being exposed to the stellar wind but moving toward the star.

The second planet is HD 63433c which is 2.67 times the diameter of Earth. The planet orbits the star every 20 days from a distance of 22 million km from the star. This distance is less than a third of the distance from Mercury to the Sun. This planet is also the same as TOI 560.01. Very hot!

Astronomers discovered the absorption of starlight by hydrogen atoms on planets. And from the Doppler shift, this gas moves away from the planet at a speed of 50 km / sec or 1800 km / h. As a result, every one billion years, planet HD 63433c loses the equivalent of 10% of Earth’s mass.

However, astronomers found no gas loss on the planet Neptune-mini HD 63433b. It seems that the planet HD 63433b is already experiencing gas loss in its atmosphere.

The Leaking Gas

The veils or gas cocoons of both planets are very thick. The gas cocoon around TOI 560.01 has a radius of 3.5 times the radius (radius) of the planet while the gas sheath on HD 63433 c is much thicker which is 12 times the radius of the planet!

However, from the gas movement rates of the two mini-Neptune planets, which are 20 km/s at TOI 560.01 and 50 km/s at HD 63433c, it seems that the planet’s gravity is not strong enough to hold the gas. As a result, there was a large loss of atmosphere and the mini-Neptune planet was transformed into a super-Earth planet.

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