Von Braun revisited

Werner von Braun is not noted for his epigrams, but perhaps he should be. Perhaps one of his best was “You must accept one of two basic premises: Either we are alone in the universe, or we are not alone in the universe. And either way, the implications are staggering”.

We’ve looked at ways of deciding how likely it is that we are alone before, using the eponymous Drake equation.  Today, we’ve gotten our first results that will help us nail down the fraction of planets that is in the habitable zone [1]. You see, we’ve found one: Gliese 581g [2], a mere 20.3 light years away.

What is it like? It orbits a red dwarf [3], which looks like a dull, glowing ember in the sky [4]. Because Gliese 581g orbits very close to its sun (every 36.6 Earth-days), the planet has become tidally locked, like the Moon. Thus, the sun stays in one place [5] and it always appears to be the same “time”. If you walk toward the day side, the sun rises into morning. If you walk toward the night-side, the sun sets into evening.

That makes the day side very hot (as much as 160 F) and the night side very cold (as much as -25 F). Thus, at ground level there will always be a wind blowing from the night side to the day side. Water will freeze out on the night side and form great glaciers that slowly flow over into the twilight lands, where they melt into vast rivers. The rivers will flow to the day side and evaporate to start the cycle over again [6].

Could we live on Gliese 581g? Probably. It has about three times Earth’s mass. But that doesn’t mean that it has three times Earth’s gravity.  If  Gliese 581g has the same average density as Earth [7], then it would have a radius of ~9200 km. That gives it a surface gravity of 1.44 g (14.2 m/s^2).  If Gliese581g has an average density equal to that of the mantle [8], then it would have a radius of ~10,800 km which gives it a surface gravity of 1.05 g (10.3 m/s^2). So walking on Gliese 581g would be like walking around with either a couple of weights or a small kid on your back.

In effect, we have extended our neighborhood to beyond the Sun. And today we have received evidence that the neighborhood is full of houses; whether they are filled with life is something that we will discover later. Just knowing that there are other houses is joy enough for today.

John

[1] I.e., the region around a star that gets enough sunlight to permit liquid qater without getting so much that all you get is steam.

[2] Gliese 581 is the name of the star (it is the 581st star in the Gliese catalog). “g” indicates that this is the seventh planet found orbiting that star. Using this naming convention, Earth is Sol a.

[3] No, not that Red Dwarf!

[4] Imagine the colors of sunset at noon. At least the photography would be good!

[5] It might actually wobble a bit, due to various orbital considerations. But this is a broad paintbrush I’m using.

[6] Sounds like the setting for a really cool space opera, doesn’t it?

[7]  Tha’s ~5.5 gm/cc. This is unlikely as the Earth’s density is anomalously high for terrestrial planets. But it gives a good upper limit.

[8] That’s ~3.4 gm/cc; interestingly, that is also close to the density of an S-type asteroid. This is a little more likely, and it provides a good lower limit.

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