New Discovery: Super Earth in a Nearby System

[throwback]

As astronomers are gaining confidence in spotting exoplanets, they are looking at old data and seeing if they can spot planets in the date that was not initially discovered. One of these planets may be orbiting HD 40307, a star that is located 44 light years away. This Super Earth planet lies in the habitable zone of its star. The scientists who "discovered" this planet believed it may have the possibility of life and may have a day/night cycle like our Earth. Even if this discovery proves false, I like that scientists are starting to develop methodologies that will permit them to see planets that are much farther out from their stars.

http://www.latimes.com/news/science/...,2600859.story

http://en.wikipedia.org/wiki/Hd_40307

[Ar-Pharazon]

Seven times the mass of Earth. How much bigger could a planet be than Earth & not have too much mass/gravity so as to be uninhabitable?

[throwback]

According to Wikipedia, the maximum mass for a Super-Earth planet is 10 Earth Masses. This is 69% of the mass of Uranus.

http://en.wikipedia.org/wiki/Super-Earth

[gturner]

Pretty big. If you rearrange Newton's law of gravitation (F=G*m1*m2/r^2) and apply it to surface gravity, assuming the planet is a perfects sphere with a given specific gravity (not gravity-gravity, specific gravity where water = 1.0), then the formula for surface acceleration in Earth G's is:

A (in Earth G's) = 0.1814 * specific_gravity * Earth_radii

The Earth has a specific gravity (density) of 5.513, and of course a radius of 1 Earth radius, so the formula spits out 1.00 G's.

Basalts and granites have a specific gravity of around 2.7 to 3, so if we didn't have the big iron core we could have a planet with half our specific gravity (2.75 instead of 5.5) and twice the radius. That would give us four times the surface area and eight times the volume (area=4*PI*r^2, volume=4/3*pi*r^3), but since we'd be half as dense, we'd only weigh four times as much. If you're willing to use that same light density and let the surface gravity go up to 2 G's, then the radius can double again, multiplying the mass by eight more, so we'd have 32 times Earth's current mass, twice the gravity, and sixteen times as much surface area.

If you're willing to live at 4G's (would fish even notice?) such a planet would have 256 times more mass than Earth, and 64 times as much surface area.

[ThunderAeroI]

So the answer is a planet 10 times as massive as earth wouldn't have nearly 4g.

[gturner]

If it had the same density as Earth (iron core) so the density is 5,500 kg/m^3, then the radius and surface gravity would be 1.58 times that of Earth (cube root of four). 1.58 G's wouldn't even be that uncomfortable, and it would be like walking around with a backpack (a 75 pound pack for a 140 pound person).

For reference, here is the density of some known planets:
Mercury 5.427
Venus 5.204
Earth 5.513
Mars 3.94
Jupiter 1.33
Saturn 0.687
Uranus 1.27
Neptune 1.638

Neptune's surface gravity is only 1.14 G's even though it's 17 times as massive as Earth, with 15 times as much surface area. Uranus surface gravity is 0.886 G's, it's mass is 14.5 times Earth's, and it's surface area is 16 times larger than Earth's.

Ignoring Jupiter and Saturn, our own solar system has planets whose density varies from ours (the highest in the system) down to only 23% as much, and since surface gravity is a linear function of density and radius, and mass a function of the cube of the radius, the surface gravity goes up with the cube root of the mass.

If you take 1.5 G's as the limit where a human would feel resonably comfortable, and the density of Uranus as the lower limit of likely density while still having a habitable surface (not a gas giant planet), then the upper end would be a planet with 6.52 times the radius of Earth, 42 times the surface area, and 64 times the mass.

So the "giant" Earthlike planets in the habital zone aren't ruled out as good places to live until the mass is quite extremely large. Unfortunately that also means likely candidates' habitability will come down to surface pressre and temperature, and those can't necessarily be determined except by close inspection. We couldn't venture a guess as to Venus' surface pressure and temperature until we had radar data from bouncing signals off the surface.

[publiusr]

Our planet would seem to be heavier looked at from afar in that we have to account for our moon as well.

What would really be nice would be a Pluto Charon type deal with two Earth mass planets rotating about each other as they both orbit their star. A two-fer

[gturner]

Now, that would be bad because both planets will invariably have technological cultures, a long history of interplanetary war, slavery and exploitation, unfair trade and economic conflicts, and fundamental religious differences, and we'd get dragged right into the middle of it, just like we always do in such star systems.

[Ar-Pharazon]

Of course, any complex life that evolved in heavier gravity would no doubt be OK there.

I just wasn't sure at what point too much gravity or mass might not allow life to form, or if at some point the size & mass would make it a gas giant or something.

[gturner]

The gas giant is probably the big risk, because if the atmosphere is thick enought it's either going to create extremely high surface temperatures or the planet will be so far from the star that there probably won't be a great enough influx of solar energy to support abundant photosynthetic surface life, on which complex animal life depends.

I don't think gravity alone would rule out life until it causes secondary problems, like molten lava flows or fusion in the core, since bacteria can thrive at 400,000 G's!

CNN story on bacteria

I would think a lot of our own multicellular ocean life would likewise be largely immune to gravit (such as jellyfish) and ocean life evolving to use neutral bouancy skeletal structures doesn't seem a bit far-fetched.