Planets and Planet-hunting: The Search for Exoplanets

exoplanet
An artist's concept of the planet around the star 51 Pegasi. The planet was first discovered in 1995. European Southern Observatory

The modern age of astronomy has brought a new set of scientists to our attention: the planet hunters. These people, often working in teams using ground-based and space-based telescopes are turning up planets by the dozens out there in the galaxy. In return, those newly found worlds are expanding our understanding of how worlds form around other stars and how many extrasolar planets, often referred to as exoplanets, exist in the Milky Way galaxy.

The Hunt for Other Worlds around the Sun

Searching for planets began in our own solar system, with the discovery of worlds beyond the familiar naked-eye planets of Mercury, Venus, Mars, Jupiter, and Saturn. Uranus and Neptune were found in the 1800s, and Pluto wasn't discovered until the early years of the 20th century. These days, the hunt is on for other dwarf planets out in the far reaches of the solar system. One team, led by astronomer Mike Brown of CalTech continually looks for worlds in the Kuiper Belt (a distant realm of the solar system), and have notched their belts with a number of claims. So far, they have found the world Eris (which is larger than Pluto), Haumea, Sedna, and dozens of other trans-Neptunian objects (TNOs). Their hunt for a Planet X sparked worldwide attention, but as of mid-2017, nothing has been seen. 

Looking for Exoplanets

The search for worlds around other stars began in 1988 when astronomers found hints of planets around two stars and a pulsar.

The first confirmed exoplanet around a main-sequence star occurred in 1995 when astronomers Michel Mayor and Didier Queloz of the University of Geneva announced the discovery of a planet around the star 51 Pegasi. Their find was proof that planets orbited sun-like stars in the galaxy. After that, the hunt was on, and astronomers began finding more planets.

They used several methods, including the radial velocity technique. It looks for the wobble in a star's spectrum, induced by the slight gravitational tug of a planet as it orbits the star. They also used the dimming of starlight produced when a planet "eclipses" its star. 

A number of groups have been involved in surveying stars to find their planets. At last count, 45 ground-based planet-hunting projects have found more than 450 worlds. One of them, the Probing Lensing Anomalies Network, which has merged with another network called MicroFUN Collaboration, looks for gravitational lensing anomalies. These happen when stars are lensed by massive bodies (such as other stars)  or planets. Another group of astronomers formed a group called the Optical Gravitational Lensing Experiment (OGLE), which used ground based instruments to look for stars, as well.

Planet Hunting Enters the Space Age

Hunting for planets around other stars is a painstaking process. It doesn't help that Earth's atmosphere makes the view of such tiny objects very difficult to obtain. Stars are large and bright; planets are small and dim. They can get lost in the glow of starlight, so direct images are incredibly tough to obtain, especially from the ground.

So, space-based observations provide a better view and allow instruments and cameras to make the painstaking measurements involved in modern planet-hunting. 

Hubble Space Telescope has made many stellar observations and has been used to image planets around other stars, as has the Spitzer Space Telescope. By far the most productive planet hunter has been the Kepler Telescope. It was launched in 2009 and spent several years searching out planets in a small area of the sky in the direction of the constellations Cygnus, Lyra, and Draco. It found thousands of planet candidates before it ran into difficulties with its stabilization gyros. It now hunts for planets in other areas of the sky, and the Kepler database of confirmed planets contains more than 4,000 worlds. Based on Kepler discoveries, which were aimed mostly at trying to find Earth-size planets, it has been estimated that nearly every Sun-like star in the galaxy (plus many other types of stars) has at least one planet.

Kepler also found many other larger planets, often referred to as super Jupiters and Hot Jupiters and Super Neptunes. 

Beyond Kepler

While Kepler has been one of the most productive planet-hunting scopes in history, it will eventually stop working. At that point, other missions will take over, including the Transiting Exoplanet Survey Satellite (TESS), which will be launched in 2018, and the James Webb Space Telescope, which will also head to space in 2018. After that, the Planetary Transits and Oscillations of Stars mission (PLATO), being built by the European Space Agency, will begin its hunt sometime in the 2020s, followed by WFIRST (the Wide Field Infrared Survey Telescope), which will hunt for planets and search for dark matter, beginning sometime in the mid 2020s. 

Each planet hunting mission, whether from the ground or in space, is "crewed" by teams of astronomers who are experts at the search for planets. Not only will they look for planets, but eventually, they hope to use their telescopes and spacecraft to get data that will reveal the conditions on those planets. The hope is to look for worlds that, like Earth, could support life.