Comments on: Well, Here’s a Pretty Good Use

By: Engineer-Poet

Engineer-Poet — Fri, 18 Feb 2005 23:15:35 +0000

I disagree; such planets WILL be tide-locked. Here's my reasoning:

Tidal effects scale as the inverse cube of distance from the primary.
You have to get roughly the same total energy flux at your planet. If you replace a 5700 K primary star with a 2000 K brown dwarf, you'll have to move roughly 12 times as close (before accounting for the smaller diameter of the dwarf).
Ergo, the tides increase by a factor of ~1700 due the decreased surface irradiance alone; even if the dwarf was 1/50 solar mass you'd still have 34 times as much tidal effect as Earth sees.
If the BD is 1/4 solar diameter, cut the orbital distance by another factor of 4 and multiply the tidal effect by a further factor of 64.

So no, I can't see how a planet circling a brown dwarf and capable of holding liquid water anywhere on its surface could fail to be tide-locked, with all the complications for life that this implies.

By: Stephen Gordon

Stephen Gordon — Thu, 10 Feb 2005 12:42:12 +0000

AndrewS:

My layman’s guess is no. And for the same reason the year wouldn’t have to be superfast.

The mass of the brown drawf is so much less than our sun that a planet could be 21 times closer, but still not have locked tides.

Of course the planet itself has to be in a narrow size range. If its too big it will have locked tides. If its too small it won’t be able to hold an atmosphere.

By: AndrewS

AndrewS — Thu, 10 Feb 2005 09:07:10 +0000

Another question for any astrophysicists out there. Would planets in the liquid-water range of brown dwarfs be tidally locked?

By: Stephen Gordon

Stephen Gordon — Wed, 09 Feb 2005 09:08:49 +0000

I’m not sure how much darker it would be. A brown dwarf doesn’t shine much, but habitable planets would be about 21 times closer to their dwarf than we are to our sun (150 million v. 7 million kilometers).

I’m guessing that planets 21 times closer to their sun would revolve around their star 21 times as fast. They’d be celebrating New Year every 17 days.

Wait, that’s probably not right. Planets 21 times closer to OUR sun would have to revolve 21 times as fast. But a brown dwarf has much less mass than our star. Planets in a brown dwarf habitable zone might have a similar length year to ours. Somebody help me out with this. Engineer Poet?

Assuming life could get started in such a place (which assumes that the development of life doesn’t require radiation peculiar to our sort of sun), native life would adapt to that lighting condition. Maybe they would have huge eyes, or a sonar sense, or eyes adapted to a different spectrum.

Finding earth-like planets around brown dwarves would be easier because brown dwarves are dimmer, but also harder because earth-like planets with liquid water would have to be so close.