#Astronomers #AlienLife – Where should we look for alien life? : In the science fiction film Interstellar, astronauts leave a dying Earth in search of a hospitable planet for the human race to settle. But the first two worlds on their shortlist – deemed “potentially habitable” from a distance – turn out to be nightmarishly hostile on closer inspection.
The crew’s first stop is an ocean planet lashed by 1km-high tidal waves, while the second is a deep-frozen world choked by toxic ammonia. While Christopher Nolan’s movie is fantasy, it draws on a real-life aspect of the work done by astronomers who study exoplanets – worlds beyond our Solar System.
The search for planets capable of supporting life could answer an age-old question: are we alone in the Universe? But what do astronomers mean when they refer to distant worlds as potentially habitable, or Earth-like?
Earth-sized planet orbits neighbouring star
“When we say ‘potentially habitable’ exoplanets, that’s a term that refers to measurable qualities of a planet that are necessary for habitable conditions,” says Prof Abel Méndez, from the University of Puerto Rico (UPR) at Arecibo. Essentially, we don’t know life can exist in such places, but it might be promising.
However, two criteria dominate popular discussions of planetary habitability: first, whether it is within Earth’s general size range (and therefore has a chance of being rocky) and, second, whether it resides in what’s known as the habitable – or Goldilocks – zone.
This is the range of distances around a host star where there’s just enough starlight to keep water in liquid form on a planet’s surface. Too close to the star, and the heat will cause water to boil off; too far away and any water will freeze.
These two parameters are useful rules of thumb, but a host of other factors influence how hospitable planets are. Some of these are excluded from the conversation because of limitations in technology.
“As we learn things about what makes the Earth habitable, things like the magnetic field become really important,” says Prof Don Pollacco, who researches exoplanets at the University of Warwick. “We can’t measure the magnetic field of an exoplanet, so we just forget about it.”
But other measurable properties are relevant to the life question. To begin with, most “potentially habitable” exoplanets orbit red dwarfs, the name for a category of stars that are smaller, cooler and dimmer than our Sun.
Red dwarfs are the most numerous star type – making up some 75% of stars in our galaxy – but that’s by-the-by. The main reason they predominate is that it’s easier to find low-mass planets there.
Astronomers hunt for exoplanets in two principal ways: the radial velocity – or wobble – method relies on detecting the gravitational pull a planet exerts on its host star, while the transit method makes use of the dip in brightness when a planet crosses in front of its star.
For the wobble method, it’s easier to detect a small planet tugging on a similarly small star, than a small planet tugging on an object many times its size.
With the transit method, a rocky exoplanet passing in front of a tiny red dwarf blocks out more of that star’s light, while the signal of an Earth-sized world passing in front of a bigger, brighter Sun-like star is more likely to be drowned out by glare.