Exoplanets

Young Astronomers Blog, Volume 28, Number 23.

Prior to 1992, the number of known planets was small; there were nine. Yes, nine. It was easy to memorize their names using the mnemonic “My Very Eager Mother Just Served Us Nine Pizzas.” Then everything changed and Pluto had nothing to do with it. Astronomers began to find planets orbiting stars other than the Sun. We call these planets “extrasolar planets” or simply “exoplanets.”

There are two primary ways to find these planets.

• Radial velocity looks for a periodic wobble in a star. This wobble is measured by the Doppler shift in the star’s light and is an indication of the gravitational pull from a planet orbiting the star.
• Transit photometry looks for a slight dimming in the light from a star when a planet moves (transits) across the star’s disk.

The search for exoplanets began with a couple near misses. Back in 1988, Bruce Campbell, Gordon Walker, and Stephenson Yang of the University of British Columbia found something that looked like a planet orbiting Gamma Cephei. However, they didn’t quite believe the results and decided that the signal was caused by something else. A year later, David Latham and a few others including Michel Mayor noticed a possible planet near star HD 114762. However, the orbit and size of the hypothetical planet wasn’t quite right, so they speculated that it was more likely a brown dwarf. Both have since been confirmed to be planets.

Much to the surprise of astronomers, the first exoplanets officially discovered were found orbiting a pulsar and not a star. In 1992, Alexander Wolszcan and Dale Frail discovered three planets orbiting pulsar PST 1257+12.

Three years later, Michel Mayor and Didier Queloz of the Geneva Observatory discovered 51 Pegasi b orbiting a Sun like star. This was another surprise. The planet is a “hot Jupiter”. It is around ½ the mass of Jupiter and orbiting its star every four days at a distance much closer than the orbit of Mercury. According to the theories of planet formation, large gas giants should form far out in their solar system. They shouldn’t be found this close to a star. But it’s there and in 2019 Mayor and Queloz were awarded the Nobel Prize in Physics for their discovery.

Once astronomers knew what to look for, other exoplanets were soon found reaching around thirty by 2000. Over the next few years, the rate of discovery increased to about thirty per year by 2006. Early on most exoplanets were discovered by measuring their star’s radial velocity.

Beginning in 2006, the search for exoplanets took to the sky with several satellites. In December 2006, the Convection, Rotation and Planetary Transit satellite (CoRoT) was launched. It discovered over twenty new exoplanets between 2007 and 2011.

From 2009 to 2018, the Kepler Telescope became the primary tool for finding new exoplanets. During Kepler’s initial mission, it monitored 150,000 stars near the constellations Lyra and Cygnus. The transit method was used to identify exoplanets. First, Kepler exoplanet candidates were identified and cataloged. Then, after a follow up confirmation, many of the candidates became official exoplanets. In 2013, Kepler experienced issues with its tracking wheels and couldn’t keep its orientation. A second Kepler (K2) mission was approved in 2013 extending Kepler’s life through 2018.

With Kepler, the number of exoplanets surged. By 2014, the number was closing in on 2,000. Around 1,500 new planets were discovered in 2016 alone. Over its two missions, Kepler itself discovered between two and three thousand exoplanets. Today, the official number of exoplanets is close to 4,300 (as of November 2020). The folks at the NASA Exoplanet Archive have a short video showing this growth over time through 2018. More up to date charts can be found on NASA’s Exoplanet Archive website.

In April 2018, the Transiting Exoplanet Survey Satellite (TESS) was launched from a SpaceX Falcon 9 rocket. TESS, like Kepler, utilizes the transit method to detect exoplanets. Unlike Kepler, TESS will survey the entire sky and search for exoplanets around 200,000 stars relatively near our solar system. Once identified, the potential planets will be further observed from ground-based telescopes and by the James Webb Space Telescope (JWST), which is scheduled for a 2021 launch.

TESS completed its two-year primary mission in July 2020 after finding 66 confirmed exoplanets and identifying over 2,000 additional candidates. Its extended mission began shortly after and will continue until September 2022.

The first group of exoplanets discovered tended to be big and hot. Large planets are easier to find and those close to their star have short orbital periods yielding multiple observations over a short period of time. Later smaller and cooler planets were found. Astronomers have found that exoplanets come in all sizes: including Terrestrial-size, Super Earths, Mini-Neptunes, Neptune-size, and Jovians. The larger Neptune-like and gas giants Jovians  can be either hot (close to their star) or cold (farther away). The University of Puerto Rico at Arecibo’s Planetary Habitability Laboratory has a nice chart showing the number of exoplanets by type as of mid-2018.

Keeping track of all the exoplanets is no small task. The good news is that there are some online catalogs available including The Extrasolar Planets Encyclopaedia, NASA’s Exoplanet Archive, and NASA’s Exoplanet Catalog. To track exoplanets on your smart phone, you might try Exoplanet by Hanno Rein. If you want to take a virtual trip among all the exoplanets, try NASA’s Eyes of Exoplanets.

One of the ultimate goals of the search for exoplanets is to find an “Earth like” planet with liquid water and an atmosphere. However, we’ll leave that discussion for a future article.