The first is astrometric, precise stellar positions are determined to try and detect the gravitational tug of an orbiting planet by a shift in the apparent position of the star relative to its neighboring stars.

The second is spectroscopic, where accelerations caused by planetary gravity are detected as Doppler blue or red shifts in the star’s spectrum.

The third is the transit, or eclipse method; where a star’s brightness dims a tiny amount when its planet passes between us. This method, unfortunately, only works when the ecliptic (orbital) plane of the planet is directly in our line of sight. If the planet passes above or below the stellar disc we will never know its there.

Each of these methods favors certain solar system geometries and requires different observational programs. For example, large planets orbiting very close to small stars are easier to spot. So the types of planets we have discovered so far are not a fair sample of the type of planets that are probably out there.

Earth-type planets orbiting sunlike stars in reasonable, well-behaved orbits are particular difficult to see. Large gas giants in highly elliptical orbits around small stars tend to be represented misproportionally. Another “selection effect”.

If the material is porous and fluffy, like a gas, the planet will be bigger and surface gravity will be less. If it is dense and compact, like stone or metal, the planet will be smaller and surface gravity is higher–assuming the total mass is identical in both cases.

Newton’s laws allow us to “weigh” planetary systems because their measurable orbits are determined by their masses. But the motions are measured in different ways. Planetary gravity can cause a star to shift in position from side-to-side as its planet orbits, But this is motion across our line of sight. Motion in the line of sight (toward and away) can only be measured spectroscopically–Doppler shift.

Unfortunately, in the real world, what we are watching is a combination of the two, and the two motions must be unraveled from one another

For example, imagine a planet in a circular orbit around a star, and the plane of that orbit is tilted 45 degrees away from us. The star will move, tugged by the planets’s gravity, from side to side (across line of sight), and towards and away from us (in line of site). THe former motion is detected by precisely measuring the star’s position relative to surrounding stars. The latter is measured by noting the blue and red shift of the spectrum. It isn’t always possible to do both, especially since they involve totally different observational strategies.

If they can calculate accurate mass by Doppler shift, I am totally confused by what I have read on the exo-planet sites I read.

Maybe I can find an example.

The mass of the planet can be determined from Newton’s laws, if we can measure the gravitational effect of the planet on its star. But I would imagine this would require some knowledge of how the orbital plane of the planet is oriented to our line of sight. The planet’s gravity can cause a measurable shift in position of the star at right angles to the line of sight, and it can cause the planet to accelerate towards and away from us (measurable by Doppler shifts in the spectrum). Both of these shifts are used by planet hunters, but depending on how much the orbit is tilted relative to us, one effect will usually dominate over the other.

Some of the guesstimates say the gravity could be up to 10G for K62F. There is also a strong faction that believes the Goldilocks super-Earths will be water planets

All of the speculation is clear that they don’t have the ability to determine the make up of the exo-planets in order to accurately figure mass. The next generation of planet finders will probably have that capability.

I should have linked the site that said 1.4 but they didn’t support the number in any way so it isn’t that important.

Two planets with the exact same mass might wind up with different radii, and hence, different surface gravities.

http://people.bridgewater.edu/~rbowman/ISAW/density-radius-log.jpg

I have no idea where the figure of 1.4 G came from.

So the gravitational acceleration (in terms of earth g) at the surface is proportional to the mass and inversely proportional to the square of the radius.

g(planet) = 2.57 / (1.41 x 1.41) = 1.29g(earth)

Not bad, a 100 kilo dude would weigh 129 kilos. We could live there.