Planets fail the Goldilocks test for ability to support life

PLANETS capable of hosting life are said to inhabit the “Goldilocks Zone” – based on the fairytale heroine’s requirement that her pilfered porridge should be neither too hot, nor too cold.

Sadly, just like porridge, many planets are at the perfect temperature only for a small amount of time, drastically cutting the chance that life has evolved on them, scientists have discovered.

Experts from Nasa Goddard’s Space Flight Centre in Maryland found that the planets fall in and out of the habitable zone over time, as a star’s brightness and temperature changes. Until now, it had been assumed that temperature remained relatively constant.

It means the number of worlds that could contain alien life has been vastly overestimated, scientists say. Dr Noah Tuchow, of the Nasa Goddard Space Flight Centre, told New Scientist magazine: “If life cannot exist on these planets, it might have major implications for the abundance of life in the universe.”

Scientists say that some planets reside in the Belatedly Habitable Zone (BHZ) – meaning that although they are now capable of supporting life their history might still make it impossible. Those born closer to their star may have all of their water boiled away before they enter the Goldilocks zone, Dr Tuchow said.

For those born further away, any water is likely to take the form of difficult-to-melt glaciers.

Experts at Nasa believe that between 29 and 74 per cent of planets in the habitable zone actually belong to this class of “belatedly habitable planets”.

Earth formed around 4.5 billion years ago, yet life did not begin to evolve until around 3.7-3.9 billion years ago. Scientists believe it took hundreds of millions of years of bombardment by asteroids and meteorites – which brought water and chemicals – before life was sparked.

Dr Tuchow added: “A planet’s history dictates its current potential to host habitable conditions and life. “Whether these planets can retain their volatiles (such as water) and support habitable conditions is unclear.

“Since BHZ planets comprise a large portion of the planets we expect to survey for biosignatures with future missions, the open question of their habitability is an important factor for mission design, survey strategies and the interpretation of results.”

Astronomers have estimated there could be as many as 40 billion Earthsized

planets orbiting in the habitable zones of Sun-like stars and red dwarfs in the Milky Way alone.

Our nearest known exoplanet that is orbiting in the habitable zone of its star is Proxima Centauri b, located about 4.2 light years away in the constellation of Centaurus.

Scientists believe life could be found in areas outside the habitable zone, sustained by other energy sources such as tidal heating or radioactive decay.

Saturn’s moons Titan and Enceladus are both thought to have liquid water oceans beneath their icy exterior which could harbour life.

Last December, data from Nasa’s Cassini spacecraft proved that Enceladus contains phosphorus, a key ingredient in the formation of DNA.

Scientists had already discovered carbon, hydrogen, nitrogen, oxygen and sulphur on the Moon, meaning that Enceladus now appears to meet all of the criteria for a habitable ocean.

Huge plumes of water vapour which erupt through cracks at Enceladus’s South Pole have been found to contain salts, methane and a variety of complex organic molecules that could indicate life beneath the surface.

The study has been accepted for publication in