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Astronomers are making progress in finding planets that broadly resemble our own.
Extraterrestrial life is a genuine scientific possibility. Earth proves that at least one habitable planet, even a planet with technological intelligence, can exist. Astronomers have discovered more than 5,000 exoplanets-worlds circling other stars-and that number increases weekly. Now the search for habitable worlds is on, pushing our data and models to the limits.
Most known exoplanets have been detected indirectly, primarily using two techniques. The transit method measures the slight dimming of a star that occurs when a planet passes in front of it. The radial velocity method measures tiny shifts in a star's light spectrum caused by the back-andforth gravitational pull of orbiting planets. The amount of light blocked during a transit indicates the size of a planet. The intensity of a radial velocity change indicates a planet's approximate mass. When combining the two methods is possible, the results reveal the planet's density. And that is where our knowledge of most exoplanets ends.
To complement the search for Earthlike planets, I have created an Earth Similarity Index (ESI) that indicates how much planets resemble Earth in their gross physical properties. The ESI is a number between zero (no similarity) and one (identical to Earth). It is not a direct measure of habitability, but rather a fuzzy comparison between a selected set of planetary properties with Earth-primarily size, mass, and insolation, the amount of radiation received from the planet's star- because those details are often all that we are able to measure for exoplanets. Worlds with high ESI values are not necessarily more habitable, because so many other factors influence a planetary environment. Venus (ESI = 0.78) and Mars (ESI = 0.65) are similar in size and insolation to Earth, but they have starkly different surface conditions.
Most known exoplanets have ESI values well below that of our Moon (ESI = 0.60). Tinere are a few exoplanets with ESI values close to one. Maybe their surfaces are genuinely similar to Earth's, with oceans and continents. Then again, they could be unsuitable for any life because they lack other life requirements, such as water, or have harmful or toxic conditions. We don't know how likely it is that superficially Earth-like planets truly resemble...