How Extrasolar Planets Are Discovered
The universe is both ancient and infinite, and at that, constantly birthing and destroying stars and star systems. With these nearly immeasurable parameters, you would think humankind had ample opportunity to encounter other, extraterrestrial lifeforms. However, the recoverable history so far states otherwise.
So, does that mean we are alone in this universe? We may be millennia away from answering that question, but fortunately, science has taken its first major step in finding out.
Through the work of Copernicus, Galileo, and Kepler, we have found out that the sun is the largest, most massive body in our solar system, making it the grand centerpiece around which all the planets revolve. The stars in the sky are not unlike our sun, spheroidal celestial bodies that consist of plasma held together by their own gravity. Based on these facts alone, it is easy to conclude that other ‘suns’ may have other ‘earths’ revolving around them.
Here is where extrasolar planets come in…
Extrasolar planets, or exoplanets, are planets found orbiting star systems other than the sun. Not all stars have planets — observable ones, at least — but all planets have stars. Planets are born from the very planetary nebulae that shape a star system, and physics dictates that, according to the law of the conservation of angular momentum, they revolve around their parent star.
How did we manage to find so many of them?
About 4,200 exoplanets have been found over the course of two and a half decades, since Michel Mayor and Didier Queloz announced the discovery of 51 Peg b, aka “Dimidium”, a fast-moving giant orbiting the main-sequence star 51 Pegasi, in 1995. A large part of this whopping amount were discovered using two methods: radial velocity and transit photometry.
Radial Velocity: The Wobbling Stars
As a middle-schooler, you were probably taught that the planets revolve around the sun. But that isn’t actually the case. If you ever paid your physics teacher any attention, you would know that two objects with mass would interact with each other, forming an equilibrium point between the two proportional to their masses.
Stars, and the rest of the star system, also orbit around each other from a common barycenter. Granted, our sun’s mass occupies more than 99% of the solar system’s, placing the barycenter within the radius of the sun more often than not, but this means the sun still moves in a pattern akin to a wobble. This wobble is observable using Doppler spectroscopy, where a star’s color undergoes miniscule changes as it moves closer (blueshift) or further (redshift) with respect to the observer.
This shift is a telltale sign of — you guessed it — an exoplanet. The radial velocity of a “wobbling” star is deduced from the displacement in the parent star’s spectral lines due to the Doppler effect. These variations are then measured using the binary mass function in order to confirm the presence of the planet.
Read about Doppler spectroscopy and the binary mass function.
Transit Photometry: The Mini Eclipses
This second method is much easier to understand when you analogize it with an eclipse much like the sun’s when it is blocked by the moon. When a planet passes directly between its parent star and the observer, an astronomical phenomenon also known as a transit, the observed visual brightness of the star drops by an amount proportional to the relative sizes of the star and the planet.
If a star has a periodical, momentary decrease in its relative brightness, it is thought to have a companion of some sorts — in this case, an exoplanet.
The graph of the star’s relative brightness during a transit can also be used to obtain a lot of information regarding the exoplanet: its radius, its period of revolution, and its orbital plane’s inclination with respect to the observer’s line of sight.
Try finding your own exoplanet using the transit photometry method in Harvard’s Laboratory for the Study of Exoplanets.
Other Methods
There are a multitude of other methods when it comes to finding exoplanets, although they are probably not as practical, or popular, among scientists and astronomers dabbling in the field.
Based on the image above, radial velocity (red) and transit photometry (green), is the most widely-used method when it comes to exoplanet detection, and by a huge margin as well. However, there are quite a few runner-ups:
- Direct imaging (blue): especially large and hot planets that are widely separated from its parent star may emit intense infrared radiation that are caught by images made in the infrared.
- Microlensing (orange): when a distant star or quasar is aligned with a massive, compact foreground object, its light may be bent due to the latter object’s gravitational field, leading to an observable magnification.
- Transit-timing variation (purple): in a star system known to have an exoplanet using the transit photometry method, variations in the timing of the transit (due to interactions with a celestial body other than its parent star) may be used to detect additional, non-transiting planets.
Watch helpful animations on microlensing and transit-timing variation.
Let’s talk about Hot Jupiters.
Hot Jupiters are a class of gas giant exoplanets — much like our own — but orbits much closer to the sun, with orbital periods shorter than ten days. Why bring this into spotlight? Because they are reminiscent of everything wrong with our current methods of exoplanet discovery.
The radial velocity method allows us to find an exoplanet’s mass, whilst the transit photometry method allows us to find its radius, period, and inclination. But these methods have their limitations, too.
For instance, the radial velocity method is only accurate for planets with orbital planes that happen to line up with the observer’s line of sight. When that is the case, the measured variation in the star’s radial velocity is the true value. Otherwise, astronomers need to further correct it with astrometric observations that track the star’s movement across a plane perpendicular to the observer’s line of sight.
The transit photometry method has two major disadvantages. Firstly, for this method to work, there must be a transit observable from the astronomers’ vantage point. Meanwhile, the probability of a transit for a planet orbiting a Sun-sized star is 0.47%. Secondly, this method has a high rate of false detection. According to a 2012 study, the rate of false positives for Kepler mission transits could be as high as 40% in single-planet systems.
With these limitations in mind, Hot Jupiters make up a large number of exoplanets discovered. It is large and relatively close to its star, increasing the likelihood of transit, and also massive enough to cause observable gravitational effect on its parent star (and in turn, the largest changes in radial velocity). Unfortunately, Hot Jupiters are clearly uninhabitable due to their high temperatures and extremely short revolution periods, and we’re looking for the opposite.
We get it, exoplanets exist. But why look for them?
With so much left to learn even within our solar system, why should we care for planets orbiting faraway stars? Scientists would say it opens up new horizons in humanity’s constant search for habitable planets. Today’s top contenders within our solar system are Mars, Jupiter’s moons Callisto, Ganymede, and Europa, Saturn’s moons Titan and Enceladus, and the dwarf planet Ceres. But these candidates require significant amounts of terraforming.
Somewhere in a faraway star system governed by a sun-like star, there may exist an Earth lookalike, with the same internal structure, atmosphere, distance from the sun, and even similar lifeforms. Here is a list of potentially habitable exoplanets found so far.
Okay, that might sound a little too far-fetched, considering the age of hyperspace jumps and spacetime alterations may be far off into the future. But more than that, exoplanets teach us how planets could be like in a star system with hotter, cooler, bigger, and smaller “suns”. And with this information, we can better understand the goings-on of our own solar system, and maybe even predict what happens to it when the sun eventually turns into a red giant, swallows Mercury and Venus, and destroy all life on Earth.
Don’t panic — it definitely won’t happen in your lifetime! For now, let’s kick back and leave the worrying to the astronomers.
Afterthoughts: are we alone in the universe?
I certainly hope not. But so far, too little could be found about exoplanets to state otherwise. Some of them are convincingly earthlike, with oxygen atmospheres and large bodies of water, like the infamous exoplanet TRAPPIST-1e in the TRAPPIST-1 star system. Unfortunately, as for proof of actual lifeforms, there is nil.
Let’s hope that, by studying more of exoplanets, we could someday hear the first signs of good news.
