Until very recently, we thought that our solar system is the prototype, along which other planetary systems should be built. We thought there were two classes of planets: solid worlds, which we find clustered in the inner regions, and gas giants that are farther away. Beginning in the 1990s, we began to detect planets near other stars, and then we found out that our solar system is not entirely normal. In a new paper that was accepted for publication this week, two astrophysicists at Columbia University attempted to figure out why.
It turns out that having small, solid planets in the inner solar system and large gas giants in the outer solar system is not entirely normal. Gas giants and rocky planets can be found everywhere, and large planets have exactly the same chances of being closer to their star as small ones. The planets we have found have shown that nothing prevents gas giants from becoming "hot Jupiters", even more - they do so quite often. The second surprise is even more surprising, and for it is worth the pioneering work of NASA's Kepler space observatory. Although solid worlds the size of Earth - both larger and smaller - are as common as worlds the size of Neptune and Jupiter, there is a third class of planets, the most common of all. Between the sizes of Earth and Neptune, there is an option that we overlooked: a super-earth (or mini-neptune). And as it turned out, the super-earth is larger than any other planet.
The first question that arose for us: why is this class of amazing worlds so densely populated? But as our models of planetary formations near stars improved, we began to see that along with the surviving planets, a smooth distribution appeared. Worlds that were too little massive, as a rule, were absorbed, thrown out or thrown into the Sun by other bodies. As the planet's mass increased, so did the likelihood of their survival. The more massive the world - preferably three times more massive than the Earth - the more likely its gravitational pull will envelop it in hydrogen and helium. These intermediate mass worlds should be somewhere between rocky planets and gas giants. But if you look for more and more massive worlds, you will see that there are less and less of them. The universe does not spawn an excessive number of massive worlds simply because it has raw materials. It would take 317 of our planets to form Jupiter alone.
As our understanding of planetary education improved, we began to have substantive questions. If super-earths were the most common type of worlds, then what is so special about the solar system that we don't have a single super-earth? The options are interesting but disappointing:
- Young super-earths formed, but did not survive, may have been thrown out along with the migration of giant planets.
- The entire inner solar system formed before Jupiter moved outward, and the solid worlds turned out to be small, because they formed late, when all the material had already been spent.
- Our massive gas giants and the Sun grabbed the first planet-forming material, leaving no chance for super-earth.
However, using the latest developments in probabilistic forecasting, scientists Jinjin Chen and David Kipping came up with a new, interesting and complete explanation. Perhaps we were very wrong.
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In most cases, when we observed planets, we knew either the mass or the radius, but not both parameters at the same time. But without knowing one parameter, it is impossible to understand what kind of world we are dealing with, with a solid like Earth or with a gaseous like Neptune. Imagine two completely different worlds, each of which is three times more massive than Earth: one has a solid core of 2.8 Earth masses with a thin shell of gas around it, and the other has a solid core of 1.5 Earth masses and the same amount of gas in the atmosphere. The first planet will be similar to Earth, but in reality it is a super-Earth: larger, more massive and with a thinner atmosphere. The second planet will be more like a mini-neptune: 10,000 kilometers of "atmosphere" above a solid surface in all directions, and the pressure on the surface will instantly crush any life we know.
Chen and Kipping's findings make it possible to accurately draw the line between super-Earths and mini-neptune. They presented a grading scheme that far surpasses our previous dire estimates. Their variant:
- Any world weighing less than 2.0 ± 0.6 Earth is likely to be solid.
- Any world between 2.0 and 130 Earth masses will be similar to Neptune.
- Anything more massive than 8% of our Sun will be a star.
- That's all. Another classification, according to astrophysicists, would be complete nonsense.
It also tells us that most of the worlds we call "super-lands" are actually located at the low-mass end of neptune-like worlds, confirming a longstanding suspicion. For planets found by the transit method, a solid world with a mass of 2.0 Earths will be about 25% larger in radius than Earth; if more, it will almost certainly be a neptune-like world with a massive hydrogen-helium shell.
And do you know why there are no super-earths in our solar system? Because with masses of 50% and 40% of this transit threshold, respectively, Earth and Venus are exactly the super-Earths that we are looking for: solid planets with a large mass. The next "class" of planets will be neptune-like worlds, and we have three of them.
"A large number of discovered planets with a mass of 2-10 Earths are often cited as evidence that super-earths are very common and our solar system turns out to be unusual," the authors write. “However, if the border between the terrestrial and Neptunian worlds is shifted to 2 terrestrial masses, the solar system will no longer be unusual. By our definition, only three of the eight planets in the solar system are Neptunian worlds, which are the most common type of planets around other solar-type stars."
In other words, it is true that there are no planets in our solar system between two and ten Earth masses, and this in itself is rare. But this is not the best way to classify planets; they are just part of the range of Neptunian worlds, and we have three of them. It turns out that we were completely wrong about the problem of missing super-lands. If we consider it correctly, there will be two interesting conclusions: what we called super-earths does not look like Earth at all, and there is no problem, because nothing has disappeared in our solar system.
ILYA KHEL