Recently discovered planet HD 40307g around HD40307 Star system is claiming that its inside a habitual zone of a solar system. Besides It has water where life could be evolved.

I tried to calculate g of that planet. But failed!

This is because the true mass of the planet could not be found. What we have is Minimum Mass which is 7.1 times Earth mass. My question is how do I know the Maximum Mass?


It isn't possible to determine upper limits for the planet masses.

The problem is that we don't know the orientation of the system relative to us. The system has been analysed by measuring the motion of the star in the direction towards and away from us. For some planetary mass $M$ we'll see the biggest star motion if we're looking at the system edge on, because in that orientation the planet pulls the star directly towards us and directly away from us as it revolves around the star. If the plane of the system is tilted with respect to us we'll see a smaller stellar motion, and if the system is at 90 degrees to us we wouldn't see any stellar motion at all no matter how big the planet's mass was.

So it's easy to calculate the minimum masses of the planets, because to do that we assume the system is edge on with respect to us. However there is no upper limit to the mass without an upper limit to the inclination of the system.

  • $\begingroup$ On the wiki I found the true mass of planets of that stellar system is not much higher than their minimum mass. What does this mean? does this(g) applied to sixth planet too? $\endgroup$ – Shiplu Mokaddim Nov 21 '12 at 18:56
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    $\begingroup$ If you make the planetary masses very high the whole system becomes unstable because the gravitational forces are so great that everything flies apart. By modelling the system you can estimate how large the masses can be before instability sets in, and then use these as a maximum. This is described in section 7 of the Arxiv paper you linked to. However the paper doesn't set any limits on the masses because of insuffient knowledge of the orbital eccentricities. I think the claim about the masses in the Wiki article is not justified. $\endgroup$ – John Rennie Nov 21 '12 at 19:07
  • $\begingroup$ Man, I didn't know that you are in astrophysics. It was a lot useful (amazing) than I thought. +1 :-) $\endgroup$ – Waffle's Crazy Peanut Nov 22 '12 at 4:32

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