Only Earth Is a Life Haven
Here are more discoveries that make it reasonable to think complex life is rare in the universe, and might be unique to Earth.
UK Astronaut Tim Peake just returned to earth after six months in space (BBC News). His wobbly legs testify to the atrophy his muscles suffered in zero-G. Overcome by the smells of his native planet, and feeling queasy, he expects recovery to take some time. “It is going to be quite tricky for me to adapt,” he said in a news conference before landing. “It’s probably going to take me two or three days before I feel well. It will take me several months before my body fully recovers in terms of bone density.” Just half a year in space did this to him. Getting to sleep was hard and not particularly restful, Simon Archer says in The Conversation.
Muscles wither, bones become lighter just as they do during the process of ageing and the cardiovascular system and vision are affected – all from the human body’s lack of adaptation to microgravity. No wonder: we have evolved over millions of years on Earth with its comforting gravitational pull.
Astronauts are exposed to excessive radiation in space. How much? Martin Archer, a space plasma physicist, decided to find out. On The Conversation, he calculated that Peake faced the equivalent of a fatal X-ray dose up there (300 joules), but since it was spread out over months and partly protected by shielding in the international space station (ISS), he didn’t die. On the surface of the earth, we only get 0.07 joules in six months, less than four thousandths Archer’s calculation – and that’s at an altitude partly protected by the earth’s magnetic field. The implications are clear:
Of course long-term exposure is very different from a short intense burst. Only astronauts who spent the entirety of those six months on a spacewalk would in fact get this lethal dose – the ISS itself helps shield them. In practice, this shielding helps limit their overall exposure to about a year’s worth of radiation that we get on Earth per day, meaning it is still some 365 times higher. Nevertheless, it’s a staggering comparison and really highlights the challenges to manned spaceflight that we’re currently facing.
In interplanetary space, the radiation is even more dangerous. Here’s a concern that sci-fi screenwriters routinely ignore:
But for missions further away from the Earth’s protective magnetic field, cosmic rays would be more of a problem. On its journey to Mars, the Curiosity rover provided crucial data on this and it was higher than expected. A round-trip manned mission to Mars would expose the astronauts to up to four times the advised career limits for astronauts of radiation due to galactic cosmic rays.
OK, so astrobiologists will admit that life needs a planetary surface and possibly a magnetic field. What else? Science Daily adds a second rinse to the the widely-accepted requirement for water. If life is to have the complex molecules we know from living cells, water is needed to get them to fold properly. Proteins fold because the push-and-pull jostling of water molecules helps them find their native configurations quickly – assuming, of course, that the amino acids are properly sequenced. Dingping Zhong explains what his team at Ohio State found:
“Here, we’ve shown that the final shape of a protein depends on two things: water and the amino acids themselves. We can now say that, on ultrafast time scales, the protein surface fluctuations are controlled by water fluctuations. Water molecules work together like a big network to drive the movement of proteins.“
We’re down to rocky planets with magnetic fields and water. What else? Better have a quiet star. Joshua Sokol at New Scientist showed an artist conception of a star blasting a young planet with the kind of radiation sun-grazing comets get. An exoplanet named KIC 12557548b in Cygnus orbits so close to its star, he says, starspots with strong magnetic fields focus death rays at it. His description of the punishment the planet gets is not pretty.
At the planet’s surface, radiation from the nearby star is hot enough to vaporise layers of exposed rock, after which a wind of charged particles – also from the star – blows that vapour into space.
Star spots may increase the devastation. These dark patches, where the star’s magnetic field is concentrated, may act as death rays, blasting out more material when the planet passes in front of them. This could happen either because these patches emit high-energy radiation that can better chip into the rock, or because the swirling magnetic fields around them help yank the vapour into orbit.
The star is peeling away the poor planet like the layers of an onion. It’s enough to make one cry.
Venus, our planetary “twin”, is not a pretty place, either, as we found at the dawn of the space age. Here’s another factor that affects habitability: electric fields. In a video clip on Space.com, Goddard space scientist Glyn Collinson tells about his “amazing and shocking” discovery that Venus has “monster electric fields” strong enough to strip oxygen atoms from the planet. Even if Venus had started with water like Earth, therefore, it would have lost it all due to this process. His work was published in Geophysical Research Letters. In its review, Science Daily says that scientists don’t know why the electric field at Venus is so strong, five times that on Earth:
Co-author, Professor Andrew Coates of the UCL MSSL, who leads the electron spectrometer team, said, “We’ve been studying the electrons flowing away from Titan and Mars as well as from Venus, and the ions they drag away to space to be lost forever. We found that over 100 metric tons per year escapes from Venus by this mechanism — significant over billions of years. The new result here is that the electric field powering this escape is surprisingly strong at Venus compared to the other objects.
Titan loses 7 metric tons per year, by comparison. This adds another concern when assessing habitability of a planet:
Understanding the role played by planet’s electric winds will help astronomers improve estimates of the size and location of habitable zones around other stars. “Even a weak electric wind could still play a role in water and atmospheric loss at any planet,” said Alex Glocer of NASA Goddard, a co-author on the paper. “It could act like a conveyor belt, moving ions higher in the ionosphere where other effects from the solar wind could carry them away.“
Keeping score, we need a rocky planet with water, a weak electric field, and a safe distance from solar magnetic storms. What else? How about not one – not two – but three Van Allen belts? Science Daily tells about how the puzzling “third Van Allen belt” is created by a “space tsunami” of sorts. Intense solar storms create this third belt, resulting in the “transport [of] the outer part of the belt radiation harmlessly into interplanetary space.” We earthlings might suffer fluctuations in the power grid and damage to satellites, but we don’t die.
The most numerous stars in the universe are the red dwarfs. We already knew that they were unlikely hosts for habitable planets, but more bad news was just announced by Astrobiology Magazine. New calculations show that “the width of the habitable zone around M-dwarf stars is not as wide as previously thought” due to Coriolis force heating as a function of orbital radius. That’s in addition to the problem that planets orbiting M-dwarf stars are likely to be tidally locked, with one hemisphere scorched by its star and the back side freezing cold. Ravi Kumar Kopparapu at Goddard Space Flight Center is trying to refine the habitable zones of these stars, and he’s not done yet. “His proposal will update our understanding of how water vapor can absorb incoming radiation from the star,” the article ends. “This can influence the warmth of a planet and further reduce the width of the habitable zone.” Goldilocks would have to get very, very lucky with the red dwarfs.
OK, we didn’t prove Earth is unique, but these articles sure give food for thought about how perfect our planet is in every way… almost like it was designed. If the majority of stars lack the requirements to host life, how many are left?
Let’s add #13 and #14 to the list of habitability requirements:
- Galactic Habitable Zone, where a star must be located (09/29/2009);
- Circumstellar Habitable Zone, the right radius from the star where liquid water can exist (10/08/2010);
- Continuously Habitable Zone, because too much variety can be lethal (07/21/2007);
- Temporal Habitable Zone, because habitable zones do not last forever (10/27/2008);
- Chemical and Thermodynamic Habitable Zone, where water can be liquid (12/30/2003);
- Ultraviolet Habitable Zone, free from deadly radiation (08/15/2006);
- Tidal Habitable Zone, which rules out most stars that are small (02/26/2011).
- Stable Obliquity Habitable Zone (1/12/2012)
- Stellar Chemistry Habitable Zone (9/08/12)
- Stellar Wind Habitable Zone (9/19/13, 6/03/14)
- Inhabitants, creating a biosphere that can regulate the atmosphere (06/06/14)
- Rotation Habitable Zone (8/15/14)
- Electric Field Habitable Zone (this entry)
- Van Allen Shield Habitable Zone (this entry)