Nearest Sun like star has planets

Daffodil.d

Found this thought it was interesting.
http://www.bbc.co.uk/news/science-environment-20770103

BULLER

Nice article. Tau Ceti is "only" 12 light years aswell, so it's comparatively in our own backyard so to speak. I wonder what the mass of the planet in the goldilocks zone is?

Myrddin

Well of course Tau Ceti has planets, its where this guy comes from



Joking aside, I think one article mentioned it had six times that mass of Earth...does this mean the gravitational force on the surface is six times stronger?

lenny palmer

EnterNow wrote: »

Well of course Tau Ceti has planets, its where this guy comes from



Joking aside, I think one article mentioned it had six times that mass of Earth...does this mean the gravitational force on the surface is six times stronger?

I don't think so. It would be more to do with its core. The bigger the core the more gravity. But i could be totally wrong

Mickeroo

I thought gravity was a direct result of mass? The bigger the object the more pull it exerts.

Myrddin

Mickeroo wrote: »

I thought gravity was a direct result of mass? The bigger the object the more pull it exerts.

So did I, but is it directly proportional? Ie is six time the mass the same as six times our gravity?

Sonics2k

EnterNow wrote: »

So did I, but is it directly proportional? Ie is six time the mass the same as six times our gravity?

I don't think this is always the case with gravity.

I believe in this case though, the mass of the planet simply refers to its size.

I'm very much own to being corrected on this though.

BULLER

EnterNow wrote: »

So did I, but is it directly proportional? Ie is six time the mass the same as six times our gravity?

Nope its a common misconception.

Yes mass gives gravity, but planets can have more mass even though they're the same size.

Mercury for example isn't that much bigger than Europa (moon of Jupiter), yet because it has such dense iron core, it has much more gravity. (.38% of Earth g)
I think Europa has less than .13% , due to the fact its made of lots and lots of water. (and hopefully a liquid ocean beneath the surface harbouring life!)

Myrddin

Sonics2k wrote: »

I believe in this case though, the mass of the planet simply refers to its size.

As in its dimensions? Surely they have no bearing on mass?*



*may be a ridiculous question

seamus

EnterNow wrote: »

As in its dimensions? Surely they have no bearing on mass?*
*may be a ridiculous question

No, the dimensions typically have no bearing on mass. That is, something being larger in volume isn't necessarily larger in mass. Mass is a function of volume and density. The denser a body is, the more mass per square meter.

There is a gravitational difference with dimensions, since distance has an impact on the effect of gravity. So if you had a planet with the same mass as earth, but half the diameter, the gravity on the surface of the smaller planet would be greater than that on earth.

In fact, some quick calculations - gravity on the surface of the smaller planet would be four times that of earth - 39.4m/s

Plug

We also have planets around our closest stars, its only a matter of years before we find a planet in the habitable zone with water on it and an atmosphere. Bring on the JWST.

Myrddin

BULLER wrote: »

Nope its a common misconception.

Yes mass gives gravity, but planets can have more mass even though they're the same size.

Mercury for example isn't that much bigger than Europa (moon of Jupiter), yet because it has such dense iron core, it has much more gravity. (.38% of Earth g)
I think Europa has less than .13% , due to the fact its made of lots and lots of water. (and hopefully a liquid ocean beneath the surface harbouring life!)

Yep I understand all that, if two planets identical in size but one has a higher mass, then its gravity will be higher than that of its twin.

What I was asking was if this new planet is for arguments sake in or around the same size as Earth, but has six times the Earth's mass...does that mean the gravitational effect is six times stronger?

Mickeroo

EnterNow wrote: »

Yep I understand all that, if two planets identical in size but one has a higher mass, then its gravity will be higher than that of its twin.

What I was asking was if this new planet is for arguments sake in or around the same size as Earth, but has six times the Earth's mass...does that mean the gravitational effect is six times stronger?

Yeah, I wasn't referring to the planets size when I use the word mass either. This is more what I meant.

ceejay

EnterNow wrote: »

Yep I understand all that, if two planets identical in size but one has a higher mass, then its gravity will be higher than that of its twin.

What I was asking was if this new planet is for arguments sake in or around the same size as Earth, but has six times the Earth's mass...does that mean the gravitational effect is six times stronger?

Yes, at the same radius, a mass 6 times larger will exert a gravitational force 6 times larger.

[latex]F=G\frac{m_1m_2}{r^2}[/latex] using Newton's approximation

Myrddin

ceejay wrote: »

Yes, at the same radius, a mass 6 times larger will exert a gravitational force 6 times larger.

Cheers, so to the layman mass is/can be directly proportional to gravitational effect...provided the comparison bodies have the same dimensions?

[latex]F=G\frac{m_1m_2}{r^2}[/latex] using Newton's approximation

My complete inability to add letters together and make numbers mean I'm rubbish at maths

ps200306

EnterNow wrote: »

Cheers, so to the layman mass is/can be directly proportional to gravitational effect...provided the comparison bodies have the same dimensions?

Yes, but it is quite unlikely that something six times the mass of the earth has the same dimensions as the earth. That would give it one sixth of the density of the earth. The density of the earth is 5.5 g/cc, so one sixth would be 0.9 g/cc which is less than the density of water. (Water is 1.0 g/cc -- grams per cubic centimetre is the same as tonnes per cubic metre). Saturn is the only planet in our own solar system with a density less than that of water but, being a gas giant, it's difficult to decide where the surface is for the purpose of calculating surface gravity.

Among the rocky planets there is some variation in density, mostly depending on the presence or absence of an iron core. Since iron has a density of 7.9 g/cc it will skew average densities considerably higher. Our moon, for instance, is made of similar stuff to the crustal rocks of the earth, with no iron core at all, and has a density of only 3.3 g/cc. Mars has a small iron core and a density of about 4 g/cc. The earth, Venus, and Mercury are broadly similar to each other with densities above 5.2 g/cc. The outer Kuiper belt objects and dwarf planets seem to have icy compositions and much lower densities -- Pluto is 1.7 g/cc.

What does all this do for surface gravity? Well, it might be a better bet to assume similar densities for rocky planets than similar dimensions. The acceleration due to surface gravity is proportional to mass divided by the square of radius:

But for a given density, mass is proportional to volume and thus the cube of radius:

and so for a given density, the gravity is proportional to the cube root of the mass. So if you assume the same density, a planet whose mass is six times that of another will only have gravity increased by a factor of the cube root of six, in other words 1.8 times, or 80% higher.

Maximus Alexander

ps200306 wrote: »

Yes, but it is quite unlikely that something six times the mass of the earth has the same dimensions as the earth. That would give it one sixth of the density of the earth. The density of the earth is 5.5 g/cc, so one sixth would be 0.9 g/cc which is less than the density of water. (Water is 1.0 g/cc -- grams per cubic centimetre is the same as tonnes per cubic metre).

You've gone and done this backwards.

Something with six times the mass of Earth but with the same volume would be six times as dense, or 33.09 g/cm^3.

Of course this is even more unlikely to be the case, since that is considerably more dense even than solid iridium at 22.56 g/cm^3.

ps200306

Maximus Alexander wrote: »

You've gone and done this backwards.

Ta. Serves me right for tryin' to be smart!