Gas giant or Terrestrial

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Gas giant or Terrestrial

Post by Ryag Han on 23rd December 2009, 1:46 pm

There are over 400 planets discovered so far.
About what percent of them are gas giants and what percent are terrestrial planets?
So far,the gas giants outnumber the terrestrials,but it is the opposite actually.
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Re: Gas giant or Terrestrial

Post by Sirius_Alpha on 23rd December 2009, 8:36 pm

Not counting our solar system...

28 Terrestiral + SuperEarths are known. (Though for most of them, we have only minimum masses)

Out of 415 known exoplanet candidates, that comes out to about 0.067% terrestrial, 99.933% Jovian.

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Re: Gas giant or Terrestrial

Post by Lazarus on 24th December 2009, 10:22 am

What do you mean by "terrestrial"?

Silicate/iron planets or worlds that aren't gas giants?

(For that matter thanks to the IAU, dwarf planets are not planets, so presumably planets that aren't giant planets are subgiant planets? Hmmm...)
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Re: Gas giant or Terrestrial

Post by Ryag Han on 24th December 2009, 11:13 am

terrestrial= any planet not made primary out of gas (this dose not includes dwarf planets indeed)
for that matter,we have yet to fiend dwarf planets outside this solar
system.
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Re: Gas giant or Terrestrial

Post by Sirius_Alpha on 24th December 2009, 12:13 pm

If PSR B1257+12 D exists (which does not seem likely), it would probably be a dwarf planet.

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Re: Gas giant or Terrestrial

Post by Ryag Han on 24th December 2009, 1:34 pm

i have doubts that we could detect a planet so small,especially
when it orbits a pulsar.
i have even higher doubts that a planet so small could be detected in
the first place so far away without direct contact.
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Re: Gas giant or Terrestrial

Post by Sirius_Alpha on 24th December 2009, 1:36 pm

Detecting bodies around pulsars is actually extremely easy. Pulsars emit pulses in predictable intervals. Timing of these pulses allows the accuracy necessary to find even lunar-mass bodies (i.e. PSR B1257+12 A).

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Re: Gas giant or Terrestrial

Post by Ryag Han on 24th December 2009, 6:31 pm

and how do you do that?
i read about pulsar planets but i don't understand how they
detected them!
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Re: Gas giant or Terrestrial

Post by Sirius_Alpha on 24th December 2009, 7:12 pm

A planet orbiting a star will cause the star to wobble around its barycenter. The star will be closer or farther from Earth, depending on which side of the barycenter the star is on. Pretty much like with radial velocity.

When the pulsar is on the far side of the barycenter, the pulses take time to cross that extra distance to reach earth, causing an apparent delay in the pulses. Over the course of the pulsars orbit about the barycenter, the pulses seem to lag behind, then speed up, then lag behind, then speed up, etc.

Monitoring these pulsar timing variations yield the presence of orbiting bodies.

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Re: Gas giant or Terrestrial

Post by Ryag Han on 25th December 2009, 9:56 am

so its basically the same way 51 Pegasi and most exo planets were
detected ?
i thought pulsar planets would take more than that!
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Re: Gas giant or Terrestrial

Post by Sirius_Alpha on 25th December 2009, 5:21 pm

Yeah, almost the same way. The differences between the two is that pulsar timing relies in light travel delay, and radial velocity depends on the doppler shifting of absorption lines.

Both have the inclination degeneracy, and both yield only minimum masses.

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Re: Gas giant or Terrestrial

Post by Lazarus on 27th December 2009, 8:04 am

Key thing to note is that timing methods rely on the light travel time effects associated position of the star relative to the centre of mass, while RV uses the velocity (obviously).

Thus the inherent biases are different: timing favours worlds on wide orbits, RV favours worlds on close-in orbits (which give higher velocities). Also the form of the degeneracy is different: for RV, the representation m*sin(i) is an approximation that holds only where the planet mass is significantly less than the star mass. For timing, the quantity you get is m*sin(i).
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