In the last ten years, 4000 exoplanets which we currently know were discovered. In this period, the process shifted gradually from discovery to characterisation. In the future, advanced equipments will help to know more about the surfaces and atmospheres of exoplanets.
Scientists raised a question about what an advanced species would see while studying the planet. Researchers from Caltech constructed a map of what Earth would appear to alien observers with the help of Earth’s multi-wavelength data. It would also serve in studying about the surface features of exoplanets similar to Earth in the future.
The study titled “Earth as an Exoplanet: A Two-dimensional Alien Map” appeared in Science Mag journal. It is led by Siteng Fan and several scientists from Division of Geological and Planetary Sciences, California Institute of Technology and Jet Propulsion Laboratory of NASA.
For finding potentially habitable planets out of Solar System, scientists cannot observe the exoplanets directly, to understand their atmosphere and surface features. So they have to only work with the indications which show that a planet is similar to Earth.
Fan said that the present exoplanet studies have not set the minimum requirements for habitability. Researchers are not sure about whether the proposed criteria are either sufficient or necessary. Besides this, the observation techniques are not enough to confirm the habitability.
As right now we know that life exists only on Earth, scientists theorized that remotely observing our planet might act as a proxy for what a habitable exoplanet might look like to an alien civilisation. The water cycle is one of the major elements of the climate on Earth and it consists of three phases. They are water vapour in the atmosphere, clouds consisting of condensed water and water bodies on the surface of Earth.
So their presence might be considered as indications of life and habitability which could be detected from a distance. To obtain Earth’s view to foreign observers, researchers compiled 9740 images that were taken every 68 to 110 minutes in 2016 and 2017 by Deep Space Climate Observatory of NASA. They were able to capture the light reflected from the atmosphere of Earth at different wavelengths.
Images were then combined to create a 10-point reflection spectrum plotted against time, integrated over Earth’s surface. This created what Earth might look like to someone who observed Earth from several light years away for a period of two years.
Fan said that the second principal component of the light curve of Earth is related to the fraction of land of the illuminated hemisphere. The reconstruction of the map translated to a problem of linear regression. So the team found the curve parameters for land and clouds after analysis of the resulting curves and comparing with the original images. They identified those parameters related to the land and adjusted it to Earth’s rotation which generated a contour map.
Black lines represent the surface feature and correspond to the coastlines of the continents which are coloured in green to roughly represent the continents. Red denotes the shallow sections of the ocean with blue depicting the deeper areas. These representations would help the scientists to understand if an exoplanet contained oceans, ice caps and clouds. Variation of life curve is dominated by clouds, land and oceans which are important for life on Earth.
In the future, instruments such as James Webb Space Telescope would help in carrying out detailed exoplanet surveys. Ground-based instruments such as Extremely Large Telescope, Thirty Meter Telescope would carry out direct imaging of planets that orbit close to their stars. With all these advancements, researchers would be able to properly determine which exoplanets are habitable and soon find the next Earth.
Journal Reference: Science Mag