Since the first confirmed discovery of an exoplanet in 1992, there have already been discovered more than 6,000 planets beyond the Solar System. Although there are many of them, and they have very specific characteristics, there is something that unites them all. Which can have a radius smaller or larger than 1.8 times the radius of the Earth, but never that. It is like a border that can be crossed or not, but that is never stepped on.
The exoplanets that are below that limit are the super earths and those who are above subneptunes. It is not known what causes this gap, but there are two hypotheses. To confirm which of them is the good one, NASA has designed a mission. The problem is that he still hasn’t gotten funding for it.
In search of young planets. The two hypotheses that exist about the origin of this gap are related to the origin of exoplanets. Therefore, the best way to unravel the mystery is to analyze young planets. The problem is that this is not easy. Of the 6,000 exoplanets that have been discovered to date, only 20 were younger than 50 million years.
The objective of the Early eVolution Explorer (EVE) mission is to launch a ship loaded with probes specialized in detecting exoplanets around young stars. If the star is young, the planets around it must be young too, since the planet always forms after its star.
Very different exoplanets. Super-Earths are rocky planets, with a radius less than 1.8 times that of Earth. They are closer to their star than the sub-Neptunes, which are also larger, with dimensions above the prohibited radius. On the other hand, sub-Neptunes have a less rocky, more spongy appearance.
The first hypothesis. As we have anticipated, there are two hypotheses about the forbidden radius of exoplanets. The first points to the same origin. Supposedly, all exoplanets were born with a rocky core that dragged clouds of hydrogen and helium around it for millions of years. The difference between them would be that the super-Earths, being closer to their star, would receive more radiation, so the gas layer would end up being destroyed. The sub-Neptunes could preserve it, hence that spongy appearance.
The second. As for the second hypothesis, it points to the possibility of planets clinging to water during their formation or not. Super-Earths are located between their star and what is known as the snow line. This is a line above which water can freeze. In this case, not only does it not freeze, but the water receives so much heat from its star that it ends up evaporating. If water is in the form of vapor, it cannot join the “pieces of the nascent planet.”
That leaves them only made of dry rock. On the other hand, the sub-Neptunes are further from the snow line. Water can freeze, so it becomes bricks that can be incorporated into the planet in formation. It is bigger, because it not only has rock, it also has water. Furthermore, that water condensed around it gives it the cottony appearance that makes it different from a rocky planet.
Artist’s concept of an aquatic world
The handicap of young stars. We have already seen that to unravel the mystery of the forbidden radium we must study young planets. We have also understood that the best way to do this is to look around at young stars. The problem is that these have such intense activity that fluctuations in their brightness can occur. associated with flaresnot an exoplanet orbiting it. In short, many false positives can occur.
Three sensors. To solve this problem, EVE would be equipped with three sensors. The first analyzes light in the near ultraviolet, the second in the visible light range and the third in the near infrared. The first is used to detect bursts from the star itself, since these emit great radiation at that frequency of the spectrum. Regarding the second, it is the type of light that is normally used to detect transiting planets, the most used tool for the detection of exoplanets. Finally, young stars emit a lot of light in the near infrared.
For all this, if a peak is detected in the near ultraviolet we will know that it is due to the activity of the star itself. If we see fluctuations in visible light we will understand that there may be a planet orbiting the star; But, to be sure, we must compare the data with the light emitted at all times by the star itself. That’s what near infrared is for.
The forbidden radio. By studying young exoplanets, we can know how their formation was and understand which of the two hypotheses is the good one. With this, in passing, we will understand why the radius of 1.8 radii of the Earth is prohibited.
30 star cluster fields, 30 days. The EVE project has been planned to analyze 30 fields of young star clusters for 30 days each. Thus, more than 20,000 young stars could be analyzed and, with them, possible exoplanets of recent formation could be found. At the moment, it cannot be done, because the project does not have financing, much less a launch date. But NASA has everything tied up. You just need that little push to unravel the mystery.
Image| Superearth on the cover and aquatic world in the text. Credit: NASA
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