If the sun evolves like its newly discovered relative, dubbed CoRoT Sol 1 -- the farthest of the so-called solar twins in the Milky Way galaxy -- all the water on Earth will boil away, they said.
"In 3 billion years' time ... the sun's radiation may increase and make the Earth's surface so hot that liquid water can no longer exist there in its natural state," team leader Jose Dias do Nascimento of the Universidad do Rio Grande do Norte said.
CoRoT Sol 1 is considered the sun's twin because of nearly identical mass and chemical composition, the astronomers said.
CoRoT Sol 1 is 6.7 billion years old, 2 billion years older than the sun, and its level of radiation is also much more intense than that of the sun, they said.
The astronomers used a spectrograph on a telescope in Hawaii to determine the star's age and its rotation period of 29 days, about a day shorter than that of the sun, the Los Angeles Times reported.
Astronomers say solar twins are important because one star may be a snapshot of the other's future.
The study will be published in Astrophysical Journal Letters.