This week the official count of known exoplanets crossed 5,000. On the one hand, there isn’t anything special about 5,000 vs 4,900 or 5,100, but on the other hand, crossing this threshold is an indication of how far we’ve come, and how quickly things will change in the future.
Although exoplanets were long thought to exist, it wasn’t until 1992 that the first exoplanet was discovered. Surprisingly, it didn’t orbit a Sun-like star, but rather a stellar remnant pulsar. The planet was discovered by measuring the shift in radio bursts from the pulsar. As the planet tugged at the pulsar along its orbit, the timing of pulses became slightly longer or shorter due to the orbital wobble of the pulsar.
A similar method was later used by optical astronomers, measuring the tiny Doppler shift of a star due to the wobble in a star’s radial velocity. For the first decade of exoplanet discovery, the pulsar method and the radial velocity method were the only way exoplanets were discovered. By 2002, nearly 90 exoplanets were confirmed, most of them orbiting main-sequence stars.
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In the second decade, exoplanets began to be discovered using the transit method. This approach measures the brightness of a star over time, looking for a slight dip in its brightness. If a planet passes in front of its star from our point of view, it blocks part of the light we see. By observing regular dips in a star’s brightness, astronomers can verify the presence of a transiting planet. This method is quite effective, and by 2012 the number of known planets had risen to nearly 800.
The first big jump in exoplanet discoveries was possible thanks to the Kepler Space Telescope. Launched in 2009, Kepler’s primary mission was to measure the brightness of thousands of stars in a small patch of sky. By 2015 Kepler had discovered more than a thousand exoplanets alone, and astronomers could now use statistical analysis to the types of planets and orbital characteristics most common in star systems.
In 2018 the Transiting Exoplanet Survey Satellite (TESS) was launched. Its mission is similar to Kepler’s but surveys an area of sky four times larger. Other missions such as Gaia are also capable of discovering exoplanets. Combined with ground-based missions, the total count as of this writing is 5,005 confirmed exoplanets and 8,709 candidate worlds. We have discovered terrestrial planets orbiting red dwarf stars, hot Jupiters orbiting close to their suns, and super-Earths unlike any world in our solar system. We’ve learned a great deal in 30 years.
But the real leap in our understanding of exoplanets is just beginning. There are potentially hundreds of billions of planets in our galaxy alone. New missions such as the James Webb Space Telescope and the planned Nancy Grace Roman Space Telescope will not only discover more planets, they will allow us to probe the atmospheres of these worlds. Just as we now know the range of mass and temperature planets can have, we will soon know their variety of skies, and whether Earth-like atmospheres are common or rare. We may even find the first signatures of life on other worlds.
Five thousand planets is an amazing milestone. But you can be sure that ten thousand worlds will be reached sooner than you think.
Reference: NASA’s Exoplanet Discovery Dashboard, 22 March 2022.