Not just water and oxygen, life thrived on early Earth thanks to another necessary ingredient: the presence of a protective magnetic field.
A new study of the young, Sun-like star Kappa Ceti has found that magnetic field plays a key role in making a planet conducive to life.
"To be habitable, a planet needs warmth, water, and it needs to be sheltered from a young, violent Sun," said lead author Jose-Dias Do Nascimento from the Harvard-Smithsonian Center for Astrophysics (CfA) and University of Rio G do Norte (UFRN), Brazil.
"The early Earth didn't have as much protection as it does now, but it had enough," added Do Nascimento.
Kappa Ceti, located 30 light-years away in the constellation Cetus, the Whale, is remarkably similar to our Sun but younger.
The team calculates an age of only 400-600 million years old which agrees with the age estimated from its rotation period.
This age roughly corresponds to the time when life first appeared on Earth.
Like other stars its age, Kappa Ceti is very magnetically active. It also propels a steady stream of plasma, or ionized gases, out into space. The team found that this stellar wind is 50 times stronger than our Sun's solar wind.
Such a fierce stellar wind would batter the atmosphere of any planet in the habitable zone, unless that planet was shielded by a magnetic field.
The team modelled the strong stellar wind of Kappa Ceti and its effect on a young Earth.
The early Earth's magnetic field is expected to have been about as strong as it is today, or slightly weaker.
Depending on the assumed strength, the researchers found that the resulting protected region or magnetosphere of Earth would be about one-third to one-half as large as it is today.
Kappa Ceti also shows evidence of "superflares" - enormous eruptions that release 10 to 100 million times more energy than the largest flares ever observed on our Sun.
Flares that energetic can strip a planet's atmosphere.
By studying Kappa Ceti, researchers hope to learn how frequently it produces superflares, and therefore how often our Sun might have erupted in its youth.
The research has been accepted for publication in The Astrophysical Journal Letters.
