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Life beyond Earth | Why do we seek planets outside the solar system?

The first exoplanet, or extrasolar planet, was found less than 30 years ago and the search for life on other worlds has only intensified

Artist impression of the exoplanet 51 Pegasi b, which orbits a star about 50 light-years from Earth in the northern constellation of Pegasus. (Credit: ESO/M. Kornmesser/Nick Risinger)

The world was introduced to exoplanets, or extrasolar plants, in January 1992. Radio astronomers Aleksander Wolszczan and Dale Frail had found two rocky planets orbiting a pulsar or a neutron star (PSR B1257+12) in the Virgo constellation some 2,300 light years away from the Sun.

Then, in October 1995, Swiss astronomers Didier Queloz and Michel Mayor said they had discovered the first planet orbiting a main-sequence star, 51 Pegasi, located 50.45 light years away from Earth in the Pegasus constellation. The planet, which was named 51 Pegasi b (officially known as Dimidium), was almost half the size of Jupiter. In 2019, Queloz and Mayor were even awarded the Nobel Prize in physics for their astounding discovery.

The journey to explore exoplanets has only intensified since. Today, there are more than 4,200 confirmed exoplanets in more than 3,000 different planetary systems. But why do we keep searching for these planets?

“We have detected small, presumably rocky planets around other stars, and some exoplanets orbit within the habitable zone of their star. But other than their mass or size, we don’t really know anything about these other worlds,” says Prof. Lewis Dartnell, from the department of life sciences, University of Westminster, London, on email.

One as yet unresolved question is just how special our own planet might be. That makes looking for signs of life elsewhere all the more important, the professor explains. “Do they (these exoplanets) have oceans, or a magnetic field, or plate tectonics, or any of the other aspects of the Earth that are thought to be important for the long-term survival of a biosphere, and especially more complex forms of life?” The answer to whether or not we are really alone in the universe depends on finding answers to such questions.

Artist impression of extrasolar planets in the pulsar, PSR B1257+12. Credit: NASA/JPL-Caltech
Artist impression of extrasolar planets in the pulsar, PSR B1257+12. Credit: NASA/JPL-Caltech

In 2009, Sara Seager, an astrophysicist and planetary scientist at the Massachusetts Institute of Technology working on exoplanets, wrote a comprehensive non-fiction account of the search for habitable worlds in her book, Is There Life Out There?: The Search For Habitable Exoplanets. She explains that regardless of whether we can reach these planets in the future, it is still important to study them.

“I believe that finding signs of life on exoplanets will change the way we see ourselves in relation to the Universe…. It will be a humbling, transformational experience,” writes Seager, who adds that when and if we find that other Earths are common and see that some of them have signs of life, “we will at last complete the Copernican Revolution”.

Prof. Dartnell says the discovery of exoplanets is just the first step. “The last three decades have seen a phenomenal boom in exoplanetary research,” he adds. “For me, the staggering achievement has been not just the number of different exoplanetary systems we have now discovered, but the incredible diversity of these new worlds… next, we need to characterize what they are like. What sorts of beasts are there within the exoplanetary zoo? And this will in turn teach us an enormous amount about our own planet, and how ordinary or special it may actually be,” says Prof. Dartnell, who authored the 2019 book Origins: How The Earth Made Us.

As the list of exoplanets continues to grow, so does the list of technological marvels used to detect them. The Spitzer Space Telescope, Kepler, Hubble and TESS (the Transiting Exoplanet Survey Satellite) have led to some of the most exciting discoveries in exoplanet exploration in recent years.

The James Webb Space Telescope, which is due to be launched in October 2021, and the Nancy Grace Roman Space Telescope, still under development, are expected to enable further headway in research. The Roman Space Telescope could tell scientists more about “rogue planets”—in a study published in the Astronomical Journal in August, researchers from Ohio State University described these as “freely floating bodies that drift through our galaxy untethered to a star”. Studying these rogue planets could help scientists understand more about how “planetary systems form, evolve, and break apart”.

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