Recently 90 exoplanets have been discovered, some by the highly sensitive radial-velocity technique of Geoffrey R. Marcy and R. Paul Butler.
How does this technique works (in laymans terms) ?
Thx.
Planets don’t orbit stars; rather planets and stars both orbit their common center of mass. Of course, since a star will generally be much (much) bigger than a planet, the center of mass will be much closer to (or even within) the star. The result is that, while the planet has a big, sweeping orbit around the star, the star has a little circular wobble of the same period.
So, for part of the star’s wobble it is moving towards the earth, and for part it is moving away from the earth. This is the “radial velocity”. We can measure this motion due to the Doppler Effect of the light emitted from the moving star, and use this measurement to calculate the presence of one or more planets.
http://cfa-www.harvard.edu/afoe/espd.html
-b
Also we have used the method to measure the ‘dip’ in brightness in a star as a planet came between us and the star. IIRC this is how the 1st exoplanet was found.
Actually, the first exo-planets were discovered around a neutron star (pulsar), which really surprised astronomers, as that wasn’t a place they expected to find planets. Neutron stars are the result of a supernova, and it wasn’t expected that a planet could survive one of those.
Pulsars, with their extremely regular radio wave pulses, make it very easy to find planets. For ordinary stars, things aren’t so easy, although the teams that specialize in finding them seem to be getting good at it. The count is now over 100, I understand.
For ordinary stars, what they actually look at is a slight shift in the spectral lines. If you look at the spectrum of the light from a star, you will see lots of lines where various elements in the star’s atmosphere has absorbed specific lightwaves. The doppler effect from the wobble caused by planets (as described by bryanmcc above) will cause these lines to shift either towards the blue end of the spectrum (when the star is approaching us) or towards the red end (when the star is moving away). If they examine these shifts over the course of at least one year of the planet, they will see a regular approach and recession caused by the planet.
But the shifts are not especially large, so they have to measure them carefully, and then do lots of mathematical analysis on them. If there’s more than one planet, the analysis will determine that too.
The dip in the brightness as the planet passed in front of the star (as described by k2dave) has been seen on one star. However, that planet was first discovered by the other method and then predicted as a possibility.
It is worth noting that inherent in the methodology is a greatly enhanced likelihood of discovering large planets, especially those orbiting close to their primaries. The first few planets discovered were planets of this type, and it became a matter of great interest that it might be such planets were much more common than was thought.
Subsequent refinements in technique have allowed progressively smaller and more distant planetary bodies to be found. Over time (allowing for observation of multiple “years” of orbit for distant planets) it may become possible to detect earth sized and smaller bodies in orbits large enough to make possible the location of candidates for terrestrial type objects.
For all I know, it might have happened already.
Tris
“It should be possible to explain the laws of physics to a barmaid.” ~ Albert Einstein ~
“You should see the place where Einstein used to drink!” ~ Triskadecamus ~
No planets have ever been detected by measuring the dip in light of the star during the transit, but such transits have been observed for planets discovered previously using the spectroscopic wobble technique.
Also, there is a present catalog of events where there was a transit of something in front of a star, but so far no one has been able to determine which, if any, of these events were due to planet transists and which were due to transits by other stars or dwarfs.
No planets have ever been detected by measuring the dip in light of the star during the transit, but such transits have been observed for planets discovered previously using the spectroscopic wobble technique.
Also, there is a present catalog of events where there was a transit of something in front of a star, but so far no one has been able to determine which, if any, of these events were due to planet transits and which were due to transits by other stars or dwarfs.
A side note:
When the first exoplanet (51 Pegasi b, now called Bellerophon) was discovered, a Catholic bishop or archbishop issued a statement saying that if there was life on this planet, their souls would need saving.
(The Church later retracted this statement; a cardinal noted that Original Sin might not have happened on other planets.)
For the record, Bellerophon is about as big as Saturn and orbits closer to its parent star than Mercury does to our own sun. Its estimated surface temperature is around 1300 degrees Kelvin. It’s not exactly a prime candidate for life as we know it.