Planets beyond our own solar system were once thought to be out of the reach of humans. The idea that we could detect worlds beyond our own or even that worlds beyond on our own might be there was questioned. But once the first few worlds were found, the flood gates were opened and the zoo of exoplanets started to reveal itself. While there is much to say about the plethora of planets we now know exist around other stars, this article will give four general fast facts about our current exoplanet understanding.
The search for exoplanets was in a hot race in the mid 90’s. Several competing teams were all staring at bright nearby stars in search of the small signal or wobble that we might detect if there was a world tugging. The star 51 Peg was a go-to target. It is a solar-type analog just over 50 light years away. It’s a northern star but close to the equator so it can be accessed by observatories in both the northern and southern hemispheres. While there was more than one team staring at 51 Peg in the year 1995, a Swiss team consisting of Michel Mayor and Didier Queloz were first to be sure of what they had found and published the discovery of a ~0.46 Jupiter mass planet with a ~4 day orbit. The news was heard around the world as it was the first secure case of an exoplanet around a star like our own. In 2019, Mayor and Queloz were awarded the Nobel prize in physics for their discovery and the subsequent explosion of exoplanet science.
A rather shocking aspect of the first exoplanet discovered was that it was a Jupiter-sized object in a very tight orbit around its star. 51 Peg b was found to have a ~4 day orbit with a distance from its star < 0.06 AU (much closer than Mercury is in our own solar system). These so-called “Hot Jupiters” were a mystery upon first finding them. Giant planets were predicted to form far from the star, beyond what is called the snow line. Forming too close to a star should have vaporized the gases and ices we know are in abundance in giant planets like Jupiter. After the discovery of 51 Peg b, an array of other hot Jupiters were discovered. They were the easiest planets to recover through the radial velocity technique. They are large objects that are tugging on their stars close-in so their observational signature is strong. At present, Astronomers are still trying to figure out how Hot Jupiters arrive at their locations. There are some theories that they can in fact form in those positions and stay stable. There are other theories that say they form farther out and migrate in after some dynamic condition pushes them from their formation spot.
There are over 4400 exoplanets confirmed to orbit their host stars. They have been found via a few well defined techniques. (1) The Radial Velocity method. Sometimes called the “wobble” method. This technique uses the idea that a planet imprints a tiny tug on its host star as it goes around and we can see the light of the star change as the planet pulls the star toward or away from us. Astronomers use high resolution spectroscopy to examine the light of a star and search for the tell tale signs of a spectral line getting doppler shifted toward us or away from us as a signature for an unseen planet. (2) The Transit method. In this case, we carefully stare at the light of a star and look for a small dip in the light as a planet passes between our instrument and the disk of the star. The Kepler Space telescope made the transit method famous and has provided the largest number of exoplanet candidates to date. (3) The Direct Imaging method. This would be the primary method of studying planets if it wasn’t so difficult to get past the bright light of the star. In this case we look directly at the planet by blocking as much of the light from the host star as possible. It only works well for very young stars since the planets are still hot and bright enough for the technique to discern them. (4) The Microlensing method. In this case we watch a crowded field of stars and wait for the effect of a star’s light getting bent and focused by the gravitational mass of an intervening object. If there is a star (and a planet) as the lens, then we see a very slight observational change as the light of a distant object gets bent and refocused by the system. (5) The Astrometric method. Similar to the radial velocity method, we can watch the position of a star by taking careful images over time and detect the tug of a planet through the slight wobble of the orbit.
The definition of the word “planet” is famously confusing. Pluto was once an accepted member of the planets of our solar system but has since been altered to “dwarf planet”. That’s an issue with the low mass definition of the word “planet”. On the high mass end, we have a similar problem. It is unclear where the word exoplanet should be used or when the word brown dwarf is more appropriate. Likely it all comes down to formation. Planets are defined as objects that form in a disk around a star. Brown dwarfs would be objects that form through the star formation process. But there might be a mass overlap between those two processes. One can find a giant planet formed in a disk of a massive star or a rogue world found in isolation. Given that there is currently no way to figure out the exact formation pathway of a newly discovered world, there is a collection of sources which defy classification as “exoplanet” or “brown dwarf”. Oftentimes we call this muddled collection “planetary mass objects”.