I had this idea for a terrestrial planet-ice giant binary system. Both planets had their origin inside the outer solar system but were moved into the habitable zone by a passers-by black hole.

The terrestrial planet, an object around the mass of Earth but with a larger radius(around 1.2 times Earth's radius), became a water world, and began to host bacterial life. However this fledgling world immediately faced a crisis caused by its larger neighbor.

The ice giant companion is around 9 earth masses and around 3.2 Earth radii, ad its composition is similar to Neptune. However, since it migrated into the habitable zone, it is no longer able to maintain its hydrogen gas. The gas escapes in sudden bursts, some expelling as much mass as Pluto from the giant. These hydrogen bursts slam into the giants companion, and are sterilizing its atmosphere.

Now, my question is, is this setup plausible? Could a terrestrial-ice giant duo even form, would these sudden hydrogen bursts be the result, and is it even possible for the water world to develop life?


2 Answers 2


The orbit

The backstory (exactly as written) is extremely unlikely. I'm not going to say impossible, but it would be a hell of a trick shot to line up. (Two sniper bullets hitting head-one kind of trick shot.)

However there is one tiny change I can make to your backstory to make it not only plausible, but more earthlike:

At some point in their history. The planets collided

It may of been billions of years ago. But if the two planets collided and transferred momentum into each, they can very easily synchronise into a binary system.

The earth and moon are actually a "binary system" (very loose meanings of the words). They orbit a center of mass (barycenter) just inside the Earth, but offset from its center.

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Forming the "binary system" from an impact is the https://en.wikipedia.org/wiki/Giant-impact_hypothesis, which explains that the earth and moon are the result of an impact. Your two planets could have collided and formed a similar system to the Earth and Moon, with your ice giant and water world.

The gassing

I can't see how it will come out in bursts - but assuming it does, hydrogen wouldn't sink to the surface - its lighter than air in the earth-like atmosphere.

The hydrogen would form an upper layer to the atmosphere. I can't see any negative effects to the life on the surface from this. Unless the hydrogen displaces all oxygen, this should be a non event.

  • $\begingroup$ Well inside the habitable zone an object with a mass of only 9 Earths and an escape velocity of 18 km/s would not be able to hold on to hydrogen at all, but I like the collision hypothesis. $\endgroup$
    – Nip Dip
    Commented Sep 7, 2020 at 4:53
  • $\begingroup$ Movies and books constantly use implausible to impossible back stories to get to the story they want to tell - so your complaint about the back story is entirely irrelevant. The melting hydrogen would occur in bursts each time the frozen giant moved closer to the sun (but it wouldn't be much of a burst, more like an increase). The real question is whether or not there's lightning on the waterworld. Boom. $\endgroup$
    – JBH
    Commented Sep 7, 2020 at 15:52

The chain of events is hard to explain from a scientific perspective:

  • the ice giant has a greater gravity than the companion, thus if the hydrogen has enough velocity to escape it, it won't be trapped by the other gravity well either.
  • the rocky planet won't be covering the entire escape surface of the ice giant, resulting in a fractional part of the flow to be intercepted.
  • I don't think having an ice giant close to a star would make it burst significantly. I imagine more of a continue and steady loss of gasses.
  • assuming that the bursts from the planet would indeed hit the planet, why would an inflow of hydrogen cause any harm to life?
  • don't forget that it's likely that before life can develop on the planet, the ice companion might have lost enough mass to have significantly changed the picture you describe.
  • there is no planetary mechanism I am aware of which can feed the burst of gas an energy comparable to a stellar wind.
  • $\begingroup$ The bursts of hydrogen could act like solar wind, stripping away sizable portions of a planets atmosphere. And if the ice giant were to be only 20% hydrogen like Neptune, it would lose around 1 to 2 Earths of mass, not enough to make a significant difference. $\endgroup$
    – Nip Dip
    Commented Sep 7, 2020 at 4:55
  • $\begingroup$ +1 for pointing out the issue with gravitation and hydrogen loss. But light I mentioned to Ash... lightning... boom. $\endgroup$
    – JBH
    Commented Sep 7, 2020 at 15:55

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