Exoplanets reach 6,000 but answer to life still elusive

The answer to the burning, centuries’ old question, whether, we humans, are alone in this universe, is nowhere in sight. But the official number of exoplanets — planets outside our solar system — tracked by Nasa has reached 6,000. The milestone comes 30 years after the first exoplanet was discovered around a star similar to our Sun, in 1995. Prior to that, a few planets had been identified around stars that had burned all their fuel and collapsed.Although researchers think there are billions of planets in the Milky Way galaxy, finding them remains a challenge. For example, while our solar system hosts an equal number of rocky and giant planets, rocky planets appear to be more common in the universe. Researchers have also found a range of planets entirely different from those in our solar system. There are Jupiter-size planets that orbit closer to their parent star than Mercury orbits the Sun; planets that orbit two stars, no stars, and dead stars; planets covered in lava; some with the density of Styrofoam; and others with clouds made of gemstones.At Nasa, the future of exoplanet science will emphasise finding rocky planets similar to Earth and studying their atmospheres for biosignatures — any characteristic, element, molecule, substance, or feature that can be used as evidence of past or present life. Nasa’s James Webb Space Telescope has already analysed the chemistry of over 100 exoplanet atmospheres.But studying the atmospheres of planets the size and temperature of Earth will require new technology. Specifically, scientists need better tools to block the glare of the star a planet orbits. And in the case of an Earth-like planet, the glare would be significant: The Sun is about 10 billion times brighter than Earth — which would be more than enough to drown out our home planet’s light if viewed by a distant observer.Confirmed planets are added to the count on a rolling basis by scientists from around the world, so no single planet is considered the 6,000th entry. The number is monitored by Nasa’s Exoplanet Science Institute (NExScI), based at Caltech’s IPAC (astrophysics and planetary science data centre) in Pasadena, California. There are more than 8,000 additional candidate planets awaiting confirmation, with Nasa leading the world in searching for life in the universe.Fewer than 100 exoplanets have been directly imaged, because most planets are so faint they get lost in the light from their parent star. The other four methods of planet detection are indirect. With the transit method, for instance, astronomers look for a star to dim for a short period as an orbiting planet passes in front of it. To account for the possibility that something other than an exoplanet is responsible for a particular signal, most exoplanet candidates must be confirmed by follow-up observations, often using an additional telescope, and that takes time. That’s why there is a long list of candidates in the Nasa Exoplanet Archive (hosted by NExScI) waiting to be confirmed.The rate of exoplanet discoveries has accelerated in recent years (the database reached 5,000 confirmed exoplanets just three years ago), and this trend seems likely to continue. Aurora Kesseli, the deputy science lead for the Nasa Exoplanet Archive at IPAC, and her colleagues anticipate receiving thousands of additional exoplanet candidates from the European Space Agency’s Gaia mission, which finds planets through a technique called astrometry, and Nasa’s upcoming Nancy Grace Roman Space Telescope, which will discover thousands of new exoplanets primarily through a technique called gravitational microlensing."Each of the different types of planets we discover gives us information about the conditions under which planets can form and, ultimately, how common planets like Earth might be, and where we should be looking for them,” said Dawn Gelino, head of Nasa’s Exoplanet Exploration Programme, located at the agency’s Jet Propulsion Laboratory in Southern California. "If we want to find out if we’re alone in the universe, all of this knowledge is essential.”