Last week NASA announced the discovery of a planet that, according to associate administrator for the agency’s Science Mission Directorate John Grunsfeld, is “a pretty good close cousin to Earth”.
Based 1,400 light years away in the constellation Cygnus, Kepler-452b is the closest match yet to our own planet, in part because of its location relative to its sun. With an orbit of 385 days, it sits right in its solar system’s habitable zone – often referred to as the goldilocks zone – meaning it could have the perfect conditions for hosting life.
Then again, of course, it might not. In our own solar system, both Mars and Venus also share the habitable zone with Earth, and neither of them have the conditions to sustain life.
Theories have existed about why this is for some time, but just a few days earlier, a paper was published that could have the answer.
Written by Matthew Jackson, associate professor in the Department of Earth Science at the University of California, Santa Barbara, and Mark Jellinek, professor in the Department of Earth, Ocean and Atmospheric Sciences at the University of British Columbia, it proposed that the key to planets in the habitable zone becoming hospitable for life was tectonic activity, and this in turn was determined by their bulk composition, ie what elements a given the planet was made up of.
“Our hypothesis suggests that among the rocky exoplanets, there’s another dial that’s important to turn when considering whether a planet is habitable or not: its bulk composition,” explained Jackson in a release about the paper.
Determining tectonic activity on Kepler-452b
We know the bulk composition of Earth, which is why Jackson and Jellinek were able to determine its role in our own planet’s tectonic activity, but could this theory also apply to Kepler-452b?
“Yes,” said Jackson when Factor asked him this question. “Keep in mind that Earth is the only planet that we know has plate tectonics. But it seems necessary to modulate climate stability over the long term.”
If we were able to determine that Kepler-452b had tectonic activity, this would move us much closer to establishing whether the planet was capable of hosting life.
Determining whether Kepler has plate tectonics over 1,000 light years distance is a big problem
“It would be a big step,” he said. “But determining whether Kepler has plate tectonics over 1,000 light years distance is a big problem!”
The distance issue also applies much closer to home. At present, we can’t even determine the bulk composition of planets in our own solar system, much less one on the other side of our galaxy.
“Unfortunately, we just don’t have the methods to infer bulk planetary composition without sample ‘in hand’,” added Jackson.
“We still struggle with the bulk composition of Mars even though we have Martian meteorites. We have no idea what the bulk composition of Venus is because of the thick atmosphere and no (known) Venusian meteorites.”
Off-Earth plate tectonics
While there is no definitive proof of plate tectonics on other planetary bodies in our solar system, some evidence has been unearthed to suggest it does happen.
Last year a paper published in Nature Geoscience showed evidence of tectonic activity on Europa, the ice moon orbiting Jupiter.
By studying images taken by the Galileo spacecraft, Simon Kattenhorn, a professor from the University of Idaho Department of Geological Sciences and Louise Prockter, assistant supervisor, Science Branch at The Johns Hopkins University Applied Physics Laboratory, were able to identify a potential tectonic plate boundary by the geological features on the moon’s surface.
However, with the technology available to us now, there is no way this sort of study could be replicated for Kepler-452b.
“Even on solar system bodies where we do have images, it is extremely difficult to tell what has been happening over a body’s history,” explained Prockter in an email to Factor. “Even on the Earth plate tectonics was not really proved until the 1960s – prior to then it was just a controversial theory.”
Galileo captured images of Europa as a resolution of 1 mile (1.6km) per pixel, so if we could somehow get images of Kepler-452b, for example by using a telescope far more advanced than those in current use, would this allow us to determine if the planet had tectonic activity?
“If images were available at the same resolutions as we have for Europa, then possibly,” said Prockter when asked this question. “But even on the Earth images would be insufficient since many plate boundaries are hidden beneath the seafloor.
“Conclusive evidence of plate tectonics on the Earth came from shipborne magnetic data. Gravity measurements might also be used, but you would need to be in orbit around a body to use that method.”
Of course, if we were in orbit, or even on Kepler-452b, the answer as to whether the planet was hospitable for life would be easier to answer through other means. Even basic visual data would tell us a fair amount, and using tectonics as a means of determining this would become rather pointless.
Establishing planetary bulk composition from afar
It seems likely that if we are to use Jackson and Jellinek’s theory to help answer the question of whether Kepler-452b is capable of hosting life, we are going to need to focus on the planet’s bulk composition, rather than its tectonic activity.
But this isn’t something we can do yet, and it is something that is going to take considerable time to get close to achieving.
“The way we can begin to get at the bulk composition as a start is to determine the mean density of an exoplanet,” explained Steve Howell, project scientist on NASA’s Kepler mission, in an email to Factor.
“Note the mean density does not tell you what the exoplanet is made from exactly nor how that material is distributed. However, knowing that elements are the same everywhere and that they form (on Earth at least) specific compounds etc. and that gravity will concentrate heavier materials in the centers of planets and lighter on the surface (as a start, plate tectonics for example changes this a bit), we can guess how a planet might be structured and what it might be made of based on the mean density.”
In the future, newer larger telescopes, and new, better instruments will allow smaller planets to have their mass measured and therefore their mean density determined
Even from afar, a fair bit of information can be gleaned to help with this calculation.
“A transiting planet has its radius known and if we can also measure its mass (from Doppler measurements using spectroscopy), we get determine density (mass/volume),” said Howell. “Not many exoplanets can have their mass measured well, as it requires a bright host star, and generally a larger (massive) planet.”
However, the technology that exists at present does not enable this to be done for a planet the size of Earth.
“Small planets, Earth size and mass, are essentially impossible to do with current technology – there are a few rare exceptions,” he said.
“In the future, newer larger telescopes, and new, better instruments will allow smaller planets to have their mass measured and therefore their mean density determined.”
There is also the possibility of establishing what the atmosphere is made up using similar methods, although this is no walk in the park.
“To determine the actual composition of the atmosphere is harder. A few very close-in orbiting large planets (hot Jupiters) have had this done via using spectroscopy,” explained Howell.
“One takes a spectrum of the star when the planet is not transiting, and one when it is and looks for very small differences that are likely due to some of the star light passing through the planet’s atmosphere.
“This is impossible today for small planets in long period orbits; usually many transits are observed and the data averaged together to get enough signal, something hard to do for a one-year orbit.”
However, there is hope. NASA is keen to improve its complement of space-based telescopes, and in 2018 will launch the James Webb Space Telescope, which will have a higher resolution and greater sensitivity, giving us views and data unreachable with the current set.
While it won’t be powerful enough to tell us much about Kepler-452b’s atmosphere, it could unearth an Earth 2.0 with an atmosphere like our own that is far closer than the Cygnus constellation.
It could even help us identify the holy grail: life on another planet.
“JWST and future NASA telescopes and exoplanet discoveries for very nearby stars by the K2 and TESS missions will help this process along, and provide possible targets to measure atmospheres and look for signatures of items such as plate tectonics (volcanic by-products) and life (free oxygen, pollution, etc.),” said Howell.
While we are a long way from establishing if Kepler-452b truly is habitable, the discovery of life on a closer planet would be even more exciting. Just think of what it would mean for humanity.