How to Find Planets in Other Solar Systems

In April of 2007 an announcement was made that a super-Earth Gliese 581 c, was found that may hold extraterrestrial life. Relatively close in galactic terms at just over 20 light years away, it nevertheless has never been seen by human eyes. How, then, was it discovered, and how do we know so much about it? How do we find planets in other solar systems?

A Look at Exoplanets

A planet located outside the solar system is referred to as an “extra-solar planet” or “exoplanet” for short. Exoplanets are just like the planets in our own solar system to some degree, with a few important variances. They are either terrestrial, or rocky, or gas giants. Some are considered “ice giants” because they are comprised mainly of water, methane and/ or ammonia but are still lumped in the “gas” giant category. As planets, as opposed to stars, these far off objects do not produce enough luminosity (in most cases) to be seen from Earth, yet we know they are there. How we know they are there is testament to the ingenuity of Earth’s scientists.

Inferring a Planet

Most of the time a planet is not “seen” or directly observed at all, but rather is inferred by its impact on other objects in space, or their impact in it. In fact, to date only one star, 2M1207b, has ever been directly imaged (and some believe that it isn’t a planet at all, but another star). One example of inferring a planet would involve studying a star for an appreciable length of time, which may result in noticing temporary periods where the star dims in the sky. It is believed that one reason this happens is that an orbiting body—most likely a planet—is passing between the star and the observer. Just from this idea alone one can see the many problems that are inherent with this method of planetary discovery. What if, for example, the view of the star from Earth is top-down (or bottom-up) in relation to a planet’s orbit? The planet may then never pass between the terrestrial telescope and the star and—using only this method—we would never know of its existence. Fortunately there are other methods employed.

Guesswork and Speculation

From examination of our own star and others nearby scientists have been able to put together a sort of star-profile, one that could suggest the possibility of planets (including their number and type) based on the classification of the star. From there scientists will then spend time examining the area around the star, looking for evidence of planets through methods such as radial velocity, the method employed to discover the very first exoplanet.

The first published claim of an exoplanet to late receive validation was made in 1988 by Canadian astronomers using the radial velocity method. Radial velocity is the measurement of light wavelengths from objects moving toward or away from the observer in a straight line. Variances in the wavelengths of stars are often made by the effect of a planetary orbit.

Final Thoughts

The reason the 1988 discoveries were not immediately accepted was because of the limitations of the equipment at that time. The evidence gathered of Gliese 581 c likewise suffers from the same problem, but the good news is the technological growth of detection devices is an exponential one.

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