The question comes to me when watching the first epsiode of the Universe, on the discovery of Pegasi 51 b the first extra-solar planet.
While I understand the method they used, what is left unexplained is how did they determine the number of days it take to orbit the sun, its size and that is a gas planet, as opposed to a Mercury, which is rocky.
Google didn’t yield anything for “how did they know Pegasi 51 b is a gas giant/planet” for me.
It’s too massive to be anything else give a starting nebula even remotely like ours.
Now the method use to detect the planet is based on watching for periodic oscillations in the star’s position. Basically the star and its planet dance around a centre of mass. The period of the oscillation is the period of the planet’s orbit.
Look at Doppler Spectroscopy for the details.
ETA –> The Kepler mission uses a different method.
I believe that they are able to use the wavelength of the light from/reflected from a star or planet to determine the basic makeup of the body. That probably tells them this is a gas planet rather than a rocky one like mercury or pluto. And the size measurement tells them it is a giant. (I think most gas planets are giant sized – if they are smaller, the lower gravity allows their gas to drift into space.)
Your own link has the answer:
That works for Solar System planets, where we can actually see the planets and take spectra of their light. But with one or two exceptions, we haven’t actually seen any exoplanets directly. And I don’t believe we’ve gotten any spectra from those exceptions.
The closest to this that’s been done is to look at light from the star when transiting planets are directly in front of the star and compare its spectrum then with the times that it’s not transiting. I think they’ve been able to get a hint of what the planet’s atmosphere is composed of, but I can’t remember details. But as SlowMindThinking indicates, transiting planets give away their size and hence their gas-giant-ness in another way.
This is just awesome! I wonder if there’s enough propellant in Kepler to do a second or third examination oriented towards differing parts of the galaxy. Currently they’re looking in the direction the sun is moving but I’d love to be able to compare those results against views directed towards and away from the galactic center.
There is a limit on just how super massive a super massive terrestrial planet can get, right?
CMC fnord!
Uranus, the least massive gas giant in the Solar System, is 14.5 times the mass of Earth. I seem to recall an educated guess that worlds whose core/mantle/lithosphere is some multiple of Earths would naturally retain enough gases through their own gravitational field to remain gas giants, leading to a “forbidden range” in planetary size: things that would have been that massive as a terrestrial planet retained gases as well and are hence above the range in mass. Wikipedia sets this figure (for the bottom of the forbidden range) at 5-10 Earth masses, a rather broad range, and one whose accuracy I cannot vouch for.
By the way, the upper range for a gas giant planet is on the order of 13-15 Jupiters (ca. 4100-4800 Earths in very round figures, Jupiter being 318 Earth masses) – above this, hydrogen fusion will begin, making it a very dim red dwarf star. (Deuterium fusion begins at 13 Jupiters, the so-called “brown dwarf” state.)
I know because I dated it’s sister.
There’s probably a practical upper bound based on how solar systems form, but it would be premature for us to try to say definitively what it is, without more data. The only solar system we have complete data on is our own, and we don’t know how typical it is.
I hear she’s so big that she has her own zip code.
She’s so big, her butt is an asterism!
CMC fnord!