Theoretically, can terrestrial planets exist beyond 20 Astronomical Units from their parent star?
Not dwarf planets, but planets at least the size of Mercury.
I know none terrestrial planets found so far from their parent star have been discovered yet, but if they would form there will they stay there or will they migrate closely to their parent star?
I have asked this as an astronomical issue not as a issue of life on another planets. I have asked if is possible for a planet the size of Mercury or Mars to orbit so far from the parent star – 20 AU, or would the parent star’s gravitation cause the planet’s orbit to migrate closely to the parent star?
For stars a few times larger than the Sun that might be possible. Here is why.
It is warmer closer to the star and colder as you go further out. This means that during the formation stage of the planetary system molecules like water, ammonia and methane tend to stay as gases in the inner systemand not produce grains and clumps and so on. And because they stay in gas form the radiation pressure from the star and the solar wind will push them outwards.
Outside the so called ice line it becomes cold enough for ice to form. In the solar system that line is at 4AU or so, say three times the outside of the habitable zone.
In the cold region the gaseous molecules can produce grains and lumps so there’s a lot of this less dense material around to accrete into planets. Jupiter and Saturn grew large enough to pull in great quantities of hydrogen and helium from the solar nebula.
But terrestrial planets can therefore only form inside the ice line: the inner planets accrete from what is available in the inner system, silicates and metals.
So to have the iceline at 20 AU instead of 4 AU, the star ought to be a few times more luminous than the Sun.
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