Since the early 1990s when the first exoplanet discovery was technically confirmed, we have thirsted to locate more and more. Now, nearly 3,600 have been confirmed, with another 4,500 awaiting confirmation! Actually, you can check mankind’s exoplanet hunt status at anytime on this wonderful Caltech Exoplanet Archive page! But, what is an exoplanet, and how do we know exoplanets exist?
What is an exoplanet?
Planet, a term coined by ancient Greeks, describes a celestial body moving in an elliptical orbit around a star.” (Dictionary.com). Simply put, an exoplanet is any given planet that orbits (revolves around) a star outside of our solar system.
Big, fast, hot, cold, tiny, if a body meets necessary criteria to be an official planet, and it is not within the grasp of our Sun’s mighty gravity, it can be an exoplanet.
Then, how do we know exoplanets exist?
But, the nearest star system to our Sun is Alpha Centauri, a triple star system 4.2 light-years (almost 25 trillion miles) away. So, how do we know exoplanets exist? After all, our telescopes do not reach that far. No man-made craft has been sent to such distances!
So, how do we know exoplanets exist at these immense cosmic distances? Actually, astronomers and astrophysicists have several methods to locate, and confirm these cosmic gems.
Transiting involves looking for small dips in the star’s light. As exoplanets orbit their stars, they occasionally come between the star and Earth. As the exoplanet crosses (transits) its sun, it ever so slightly masks the star’s light, causing the previously mentioned dips!
Plus, with impressive precision, modern science and mathematics reveal substantial detail about these technically unseen worlds! This transit discovery method has located the majority of found exoplanets so far.
In some scenarios, a planet orbiting close to its star causes a spike, instead of a dip, in the star’s light, as seen from Earth. As a result, the planet is heated up tremendously, causing thermal (infrared) radiation emissions. And, we can observe this increased heat!
However, this orbital brightness method has only helped us find and confirm only small amounts of new exoplanets. Primarily, the method only works well when planets orbit closely around their star, which is not extremely common.
As we now know, a solar system is comprised of a massive star, with planets and other space debris locked, orbiting in the star’s gravity. And, much like our Sun, the star sits right in the middle of the system’s center of gravity.
However, in some cases, planets may grow massive enough to slightly tug on the star as they orbit around. As a result, this tugging pulls the star gently out of the dead-center of gravity.
Using this gentle pulling, astronomers can gather a lot of information about distant planet’s density, composition and more! In fact, until the transit method (above) was invented, this radial velocity discovery method was the most common.
Yet, one major limitation of this method is that it typically only works for large planets––think Jupiter––small, Earth-like planets remain elusive to such phenomena.
Gravitational lensing allows astronomers to use the universe itself as a magnifying glass!
All planets have gravity. And, this gravity warps space (remember, General Relativity?), creating a lensing effect. This warped lensing allows us to magnify much more distant objects or areas.
Though, for this method to work successfully, the two objects must line up. For example, a closer and much more distant object may line up with Earth temporarily. This can allow us to use the closer object’s gravity to magnify the more distant object. Therefore, seeing the farther object much more easily and clearly!
Using our eyes
Last but not least, astronomers rarely receive the gift of spotting exoplanets the “old-fashioned” way, with our eyes!
Mainly, this discovery method requires a planet to orbit fairly far from its star. For instance, we struggle to see Mercury from Earth, simply because it remains close to our Sun. So, imagine trying to spot Mercury from a distant solar system. Close to impossible!
Similar to the radial velocity (above) method above, large and hot worlds, similar to Jupiter, make this method much more effective. The thermal radiation given off by a planet can be clearly observed from our powerful Earth or space telescopes.