Cosmic Lottery: How Many Habitable Planets?

News media ran with a suggestion that one in five stars has a habitable planet, but they didn’t read the fine print.

Here’s how it came out in the mainstream media:

• “One in five suns has habitable world: Astronomers have estimated how many of the 100 billion stars in our galaxy hosts a potentially habitable planet.” (BBC News)
• “One in Five Stars has Earth-Sized Planet in Habitable Zone: Scientists from University of California, Berkeley, and University of Hawaii, Manoa, have statistically determined that twenty percent of Sun-like stars in our galaxy have Earth-sized planets that could host life.” (NASA Astrobiology magazine)
• “How Common Are Habitable Planets? One in Five Sun-Like Stars May Have Earth-Size, Potentially Habitable Planets.” (Science Daily)

At least Science Daily’s headline was worded slightly less conclusively.  PNAS just issued a correction to the paper on which the claim was based.  That correction points out the huge error bars in making such estimates:

Estimates of the occurrence of Earth analog planets appear in several previous works including Catanzarite and Shao, Traub, and Dong and Zhu. These estimates, which range from 1% to 34%, were built upon early catalogs of Kepler planet candidates (based on less than 1.3 years of photometry). These estimates did not address survey completeness with injection and recovery or uncertain stellar radii with spectroscopy.

The paper when it came out also issued a serious caveat about what “habitable” means:

Although the details of planetary habitability are debated and depend on planet-specific properties as well as the stochastic nature of planet formation, the habitable zone (HZ) is traditionally defined as the set of planetary orbits that permit liquid water on the surface. The precise inner and outer edges of the HZ depend on details of the model.

On Evolution News & Views, Rob Sheldon and Denyse O’Leary criticized the estimate and the news media’s celebrations.

Meanwhile, PhysOrg mentioned another factor that could sterilize a habitable planet: “Cosmic rays zap a planet’s chances for life.”  The article begins with artwork of cosmic bullets, traveling at nearly the speed of light, bombarding the planet, splitting into lethal showers of high-energy particles hitting the surface.  Those particles have the power to disrupt DNA.  Enough of them would kill any living thing.

Earth gets hit by cosmic rays, too, but it has the right balance of atmosphere and magnetic field to disarm most of them.  A team of astrobiologists estimated that atmospheric thickness is the more effective of the two factors (atmosphere and magnetic field) that protect a habitable planet from getting zapped.  Astrobiology Magazine noted that Mars is probably sterilized by solar radiation, even if the 1976 Viking experiments were to be re-interpreted to permit the possibility of life: “some scientists say it’s probable that high solar ultraviolet radiation hitting the soil makes the surface sterile and hostile to life.”

The Astrobiology Magazine article went on to describe new experiments simulating the gamma radiation of cosmic rays on Martian soil.  Richard Quinn of the SETI Institute irradiated the perchlorate that is ubiquitous on Mars with gamma rays, and was able to reproduce the output of Viking’s labeled release experiment that, to some, gave ambiguous results about the possibility of life.  This strongly indicates that at least for Mars, with its “harsh radiation environment on the Red Planet, whose thin atmosphere is not enough to shield the surface from highly energetic particles bombarding the surface,” being in the habitable zone would not help make it lively.

Incidentally, one of the brightest gamma ray bursts ever recorded was observed in the direction of the constellation Leo, Space.com and Science Daily reported this week.  It was detected by the orbiting Swift telescope, “because Earth’s atmosphere absorbs the gamma radiation.”  Scientists are puzzled because it “defies astronomy theories,” Space.com’s headline reads.

On Evolution News & Views, David Klinghoffer announced that a Canadian initiative called UrtheCast (Earthcast) will soon be streaming live images from space of our beautiful blue Earth, full of life.  “It’s hard not to see this, in some karmic sense, as a rebuke to the ongoing buzz about how many habitable ‘Earth-like’ planets people think are out there sailing through the Milky Way,” he said. “The estimate has been bid up as high as 40 billion — an exercise, as we’ve pointed out, largely of imagination.”

Now we can add another factor to our list of constraints on the habitable zone. Although cosmic rays careen throughout space, it is likely there are zones of increased danger, such as the centers of galaxies or locations where gamma ray bursts are more likely to occur (e.g., wherever giant stars are concentrated).  We’re up to 11 factors now:

1. Galactic Habitable Zone, where a star must be located (09/29/2009)
2. Circumstellar Habitable Zone, the right radius from the star where liquid water can exist (10/08/2010)
3. Continuously Habitable Zone, because too much variety can be lethal (7/21/2007)
4. Temporal Habitable Zone, because habitable zones do not last forever (10/27/2008)
5. Chemical and Thermodynamic Habitable Zone, where water can be liquid (12/30/2003)
6. Ultraviolet Habitable Zone, free from deadly radiation (8/15/2006)
7. Tidal Habitable Zone, which rules out most stars that are small (02/26/2011)
8. Stable Obliquity Habitable Zone (1/12/2012)
9. Stellar Chemistry Habitable Zone (9/08/12)
10. Stellar Wind Habitable Zone (9/19/13)
11. Cosmic Ray Habitable Zone, protected by magnetic field and atmosphere (11/23/13)

Remember that failure to meet any one of these targets can be a show-stopper.  It does not rule out other winners of the cosmic lottery, but reporters need to be realistic when reporting news about “habitable planets.”  There’s much more to being habitable than simply being at the right radius from a star.  Another misleading thing about the PNAS paper was that it calculated its percentages based on sunlike stars.  Those are a small fraction of all stars, most of which are red dwarfs.  Red dwarfs are more likely to violate all these constraints.  And remember, habitable does not mean inhabited.  The origin of life remains an impossible hurdle for naturalistic ideology (see online book).

The Privileged Planet is still an up-to-date video to ponder and enjoy.