It was the news for which everyone had been waiting.
At the end of last month, the popular press went wild. A planet had been discovered that was so much like Earth it was heralded as our best bet for supporting life. Positioned 16 light years away, Gliese (or GJ) 832c was a mere hop from home and there were rumours a popular coffee shop chain had already applied for planning permission.
And this was all utterly wrong.
The journal article, published by a team led by Robert Wittenmyer from the University of New South Wales in Australia, described a world boiling under its own stifling cloud cover. With an orbit that only skirted the region capable of maintaining water, and a mass sufficient to attract a thick atmosphere, the planet was designated a likely ‘super Venus’ and unsuitable for life. What was more, it was so close to its star that it risked being in a tidal lock, with one side doused in the heat of a perpetual day while its reverse remained shrouded in night.
The answer lies in the use of a quantity denoted as the ‘Earth Similarity Index’ or ESI. As the product of the differences between the planet’s bulk properties and those of the Earth, the ESI is designed to indicate how ‘Earth-like’ a planet might be. The problem is that the resulting number is a weak comparison between the two objects and has absolutely no quantitative meaning for habitability.
Accessing the habitability of a world outside our Solar System is no easy task; a fact that lies at the heart of why the ESI is fallacious. Planetary scientists are typically working with only the mass (and sometimes radius) of the planet and the type of star it is orbiting.
The latter property can be used to define the ‘Habitable Zone’. Coined in 1959 by scientist Su-Shu Huang working at the Berkeley Astronomical Department, the Habitable Zone around a star is the location where water could exist on the surface of a planet, if that planet had a sufficient atmospheric pressure.
The caveats here are important: the Habitable Zone does not say there is water present, or that a planet exists that might be able to support life. It only marks out a region where the amount of stellar radiation would not boil nor freeze water. In short, very few planets found in the Habitable Zone of their star will be suitable for life, but if another Earth-like planet existed, it would be there. This makes the region a prime search area for future missions targeted towards habitability, including the forthcoming space telescope, JWST (predicted launch 2018), and future terrestrial planet-finders.
In the case of GJ 832c, the planet orbits just inside the inner edge of its Habitable Zone, providing the location of its edge is extended to its most generous estimate. The generosity requires scientists to include the possibility of water that is only supported during the early evolution of the planet, as is thought to have been true for Mars and Venus.
However, it is not its proximity to the edge of the Habitable Zone that kills the deal for GJ 832c. With a measured mass at least five times that of the Earth, GJ 832c is capable of attracting a thick atmosphere. Even if the gas constitution was similar to that on Earth, this would result in a high quantity of greenhouse gases that trap heat reflected from the planet’s surface. The surface temperature would therefore be kicked beyond that expected at the edge of the Habitable Zone to ensure no water (and therefore no known forms of life) exists. If that is not unpleasant enough, the higher planet mass could result in hydrogen and helium being retained in the planet’s atmosphere (they escape the Earth) to produce an utterly unusable mess.
Finally, GJ 832c is very close to its star, with an orbit of only 35.8 days. Since the star is small and relatively cool, the planet is saved from the cooked fate of Mercury, but the gravitational pull is likely to be sufficient to cause a tidal lock. In this situation, the planet will rotate an exact number of times per orbit. If that number happens to be one (as it is with the moon’s orbit about the Earth) then one side will be perpetually facing the star to create a strong temperature gradient across the planet. While this alone would not definitely rule out life, it is one more challenge on a task that is already basically impossible.
So why does the ESI come out so suggestive of a second homeland?
It is because it is not capable of taking into account any of the above factors.
The calculation of the ESI is based on four parameters: mean radius, bulk density, escape velocity and surface temperature. These are weighted by an exponent that reflects the range each property can take while maintaining a reasonable ‘Earth like’ condition, and then multiplied together to give a value between 0.0 and 1.0. Any value above 0.8 is considered a near-Earth match. GJ 832c has an ESI of 0.81.
The reason this holds no water (pun intended) boils down to three reasons:
The first is the small number of measurements that are being compared. When observing exoplanets, scientists can only measure the planet’s mass and radius, with the latter only being possible if the planet is transiting across its star. This means that only three of the four variables in the ESI can vary independently, since both density and escape velocity are calculated from the mass and radius. In the case of GJ 832c, which was detected from its star’s radial velocity, we only have a measurement for the minimum value of its mass, reducing our Earth comparison points to a poor two.
Within these three (or two) variables, a second problem arises in achieving an accurate value for the planet’s surface temperature. As with the location of the Habitable Zone, the temperature is estimated by calculating the radiation intensity from the star at the planet’s location. However —as we have just seen— this does not allow for the planet’s atmosphere. If we perform the calculation for our own Sun, the surface temperate estimates are low but reasonable for the Earth, but less than half the correct value for the closer Venus. This is due to the extra solar radiation boosting the greenhouse gases in Venus’s atmosphere, which spiral upwards to roast the planet’s surface.
The location of this ‘Runaway Greenhouse’ effect for an Earth-sized atmosphere is used to define the inner edge of the Habitable Zone, which places Venus outside it. However, the ESI calculation for Venus with its estimated temperature would give it an incredible 0.9 match with the Earth*.
“All conversations regarding habitability are worthless if we ignore the limited data present within our own Solar System,” points out Prof. Stephen Kane, a planetary scientist from San Francisco State University. “If we start defining Venusian planets as being habitable then we are no longer doing science.”
For GJ 832c, whose larger mass boosts the greenhouse effect, the discrepancy of its actual to estimated temperature is likely to be much worse.
The third problem with the ESI is that even if its parameters could be accurately and independently measured, they cannot be combined to determine habitability.
While the mass, radius and temperature certainly have a baring on the planet’s environment, they are overwhelmed by other factors that contribute to the support of life. For example, water is thought to have been delivered to the Earth by ice-rich meteorites scattered inwards by the outer planets during its formation. A different system of planets could bypass this process, allowing Earth’s twin to form but be uninhabitable to all known forms of life. Similarly, the Earth’s magnetic field protects it from harmful solar radiation, its distance from the Sun allows it to rotate hundreds of times per orbit to ensure an even distribution of heat and the Sun itself is a quiet star without violent radiation outbursts that could overwhelm the Earth’s defences. These are a small fraction of the processes that will determine a planet’s suitability for life and none of them are included in the ESI.
The upshot of this is that a planet with an ESI of 0.1 is just as likely to support life as one with ESI 0.99. In the case of GJ 832c, the ESI estimates habitability based on two measurements, the first of which (the high mass) suggests the second (the temperature) is wildly inaccurate.
The ESI is neither used nor referred to in scientific publications, but its tantalising one-number answer has caused it to propagate rapidly through the majority of science news sites. The result is an article that is at best misleading, and at worst downright wrong.
Perhaps though, the most disappointing fact about the ESI is that it detracts from the real excitement of GJ 832c. With its high mass and thick atmosphere, the planet has been declared a ‘super Venus’, for which the only other example (Kepler 69c) was discovered last year.
Kane led the characterisation of Kepler 69c as a super Venus in a paper that identifies the planetary class.
“The Kepler telescope is discovering the Venus-analogues first,” Kane explains. “Understanding how common they are will help us to decode why the atmosphere of Venus so radically diverged from its sister planet, Earth.”
* The quoted value for the ESI of Venus is 0.44, but this is from using Venus’s true temperature, not the estimate that would be made in an exoplanet detection.