Ravi Mandavi said:
additional question :
how can we predict or know that xyz planet is a Earth like planet or having a Earth like condition, when we are not abl to see it directly (if i am not wrong) ?
You're not wrong. There are only a handful (literally fewer than 10) of planets that we've been able to image directly (and even then, we can't tell much about them). The rest have been detected by indirect means.
There are a couple of different indirect methods for detecting exoplanets. One is the Doppler or radial velocity technique, in which a planet tugs on its parent star a bit due to gravity, and we can see this wobble as change in velocity of the star, by means of the Doppler effect. This gives you a lower limit on the mass of the planet (there is some uncertainty due to the orbital inclination).
The second technique is called the transit technique. If you're lucky, the orbit will be inclined such that, from our point of view, the planet passes in front of its parent star once per orbit (a "transit") and then passes behind it (an "occultation"). The light output from the star that we measure drops a bit when the planet is in front of the star, obscuring a small part of it. By measuring the drop, we can determine how much of the star's surface the planet is covering, and therefore what the radius of the planet is, relative to its parent star.
If you're lucky enough to have detected a planet *both* by the Doppler technique and by means of a transit, you can get both the mass and the radius. That let's you determine the mean density of the planet, and hence, in a vague sense, what material it is mostly made out of. I.e. is it dense enough to be solid/rocky, or is it mostly liquid water, or is it gaseous? If you have a planet that is a few Earth masses, is similar in size to Earth (maybe a factor of a few larger), then this is already starting to fit the astronomers' definition of "Earth-like."
Finally, although we can't quite do this yet, if you have a transiting planet, and you do spectroscopy (i.e. measure the
spectrum) on the light from the star, you might find that there is a difference in the spectrum when the planet is in front of the star (compared to when it is not) because the star's light is passing through the planet's atmosphere, and so you might see spectral lines in absorption that tell you what the chemical composition of the planet's atmosphere is. You could determine if it contained oxygen and other things that are conducive to life as we know it on Earth. I don't think we can quite do this yet, because our instruments are not yet sensitive enough.
) but in the future when I'm no longer a poor student I'll maybe starting doing the same as you just did!
