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Earth May Be a Second Generation Planet


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As we have developed the instruments to search for planets in other solar systems, we have noticed that there are very few planets like our own. Many are actually what we call super-Earths, which are bigger than Earth but smaller than the gas giant Neptune. Researchers at Caltech have a possible explanation for why our planet is as small as it is, and solves other mysteries too.

Our current understanding of how solar systems form describes a large disc around the star, filled with hydrogen, helium, and objects called planetesimals, which can collect together to form planets. According to the Grand Track scenario, Jupiter formed and was so massive it cleared out part of the disc, and was then being pulled into the Sun, along with the dust and gas in the disc. Saturn then formed and got pulled in towards the Sun as well, but at a faster rate, so it eventually caught up to Jupiter. At this point the two giant planets would achieve orbital resonance and that led to their orbits moving away from the Sun, but not before having disturbed the inner part of the disc. If the Solar System formed like others, and had super-Earths orbiting near the Sun, that disruption could have sent enough of those planetesimals into the super-Earths to cause them to crash into the Sun, destroying them. At this point, much of the gas and dust would have been removed from the inner disc, but if even just a tenth of the planetesimals had achieved circular orbits, there would have been enough mass for the inner planets, Mercury, Venus, Earth, and Mars to form.

Along with explaining why there are no super-Earths here, like we see in other systems, this model also explains why measurements indicate Earth formed 100-200 million years after the Sun had and why there is so little hydrogen in our atmosphere. The hydrogen-filled disc was already gone by the time the second generation of planet formation began in the inner ring.

Source: California Institute of Technology

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