Astronomers in the United States on Monday announced a milestone in the quest for life beyond Earth as they unveiled the discovery of the smallest planet yet outside the solar system.

Said to be the ‘‘most Earthlike planet ever found’’, the new planet, orbitting a dim red star in Aquarius known as Gliese 876, is the third and innermost member of a shrunken version of our own solar system. Its detection gives scientists hope that others like it, and Earth, are common in the universe.

The discovery was made using a Hawaii-located 10-meter diameter Keck telescope. On the team were scientists from the University of California, Berkeley, the Carnegie Institution, NASA’s Ames Research Center, and the University of California, Santa Cruz.

Alien-planet news has had an almost nonstop run since the first was detected 10 years ago. Most of the 150-odd planets that have been discovered since are gas giants like Jupiter. But, said team member Dr. Geoffrey Marcy, ‘‘this is different’’, the planet being made of the same materials as Earth, and perhaps owning an atmosphere.

The team had long been interested in the Gliese 876 system, about 15 light-years from here. Its two giant outer planets orbit in an unusual lockstep configuration, taking 30 and 60 days to go around, so that they line up every two months.

The new planet, according to the data, circles Gliese every 1.9 days, putting it slightly less than 2 million miles from the star. By comparison, Earth is 93 million miles from the Sun.

This planet is about 70 per cent bigger than Earth, Marcy estimated, and is made of silicates, iron and nickel, like the terrestrial planets in our own system. At such a close distance, the planet could be tidally locked, keeping the same face toward its star at all times. One would expect its to be an inferno. However, Gliese 876 is small and dim, about 1/50th the luminosity of the Sun. Marcy said that heat levels on the side of the planet facing the star would be at ‘‘chicken-roasting temperatures’’, between 400 to 700 degrees Fahrenheit (approximately 200 to 370 degrees Celsius).