The magnitude of the wobbles indicates that the planet is at least 1.35 times the mass of Earth but could easily be twice the mass of Earth.
Astronomers’ instruments are not yet sensitive enough to spot Earth-size planets in Earthlike orbits around stars similar to our sun. It is easier to detect Earth-size planets around dimmer and cooler stars known as red dwarfs, which are the most common type of star in the Milky Way.
Astronomers have in the past couple of decades discovered an abundance of star-hugging planets, far different from anything in our solar system. The Ross 128 planet is only about 4.5 million miles from the star, much closer than the 93 million miles between Earth and the sun. Even Mercury, the innermost planet of the solar system, is 36 million miles from the sun.
If the newly discovered planet were the same distance from the red dwarf as Earth is from the sun, it would be frigid. But it is close enough to Ross 128 that it absorbs warmth sufficient for liquid water, one of the requisite ingredients for life, to potentially exist on the surface. (If anything, the planet may be too warm, more like the planet Venus.)
Dr. Bonfils said Ross 128 appears to be at least five billion years old — older than our solar system — and perhaps as old as 10 billion years. The star may have been more turbulent in its youth. But even if solar flares billions of years ago stripped away the planet’s atmosphere, it could have been replenished by gases emanating from the planet’s interior, Dr. Bonfils said.