Why life might be unique to Earth

By Mark Waghorn via SWNS

Life really may be unique to Earth, according to new research.

The number of alien worlds where it could emerge has been vastly over-estimated, say scientists.

This is because few spend enough time in the 'habitable zone' - where there is likely to be liquid water on the surface.

It changes as a star's brightness and temperature evolve - rather than remaining relatively static as is often assumed.

Planets that started out either much too hot or cold may become more temperate only later in their lifetime.

Dr. Noah Tuchow, of NASA's Goddard Space Flight Centre in Maryland, said: "If life cannot exist on these planets, it might have major implications for the abundance of
life in the universe."

The team has dubbed them 'belatedly habitable planets' - as opposed to continuously so, reports New Scientist.

They found the phenomenon could apply to as many as three-quarters, with big consequences for the possibility of water on them.

Those born closer to their star may have all of their water boiled away before they enter the habitable, or 'Goldilocks,' zone.

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

We don’t know much about how life arose on Earth, so our understanding of the conditions necessary for life is vague.

But it is clear that belatedly habitable planets have more obstacles for life than continuously habitable ones.

Dr. Tuchow said: "A planet's history dictates its current potential to host habitable conditions and life.

"In this study, we focus on Belatedly Habitable Zone (BHZ) planets - that is planets that enter the habitable zone after formation due to the evolution of their host star.

"We find that between 29 and 74 percent of planets in the habitable zone belong to this class of BHZ planets depending on the timescale for the delivery of volatiles.

"Whether these planets can retain their volatiles 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."

The study was published on the science website arXiv.