This past year there have been hundreds of news reports about the discovery of so-called "Earth-like" planets.
Only a month ago, a team of researchers announced that a super-Earth class planet called GJ667C is close enough to its orange dwarf star to have surface oceans.
Two months earlier, Kepler-22b was widely reported as the smallest planet yet found to orbit in the middle of the habitable zone of a sun-like star. The planet is about 2.4 times the radius of Earth making it a big ball of rock (presumably).
The ultimate goal of NASA's Kepler mission it to give us a statistical estimate for the abundance of Earth-sized planets in stellar habitable zones across our galaxy. But I've previously cautioned that we don't know how Earth got its water, or even how much there even is inside our planet. So even if a planet is in a Goldilocks zone there's no guarantee a planet has much, if any, water.
Now let me toss out another huge qualifier: Not all rocky planets are made from exactly the same recipe. New studies suggest that the Milky Way should have a greater diversity of terrestrial planets than we can imagine. Therefore, truly Earth-like planets are an as yet undetermined subset of this population.
In a recent Astrophysical Journal Letter Jade C. Carter-Bond of the Planetary Science Institute in Tucson, Arizona, and co-investigators, report that computer simulations of terrestrial planet formation show that, chemically, they can be anything but Earth-like.
Other stars have proportionally different abundances of elements. And, observations of white dwarfs show that they are polluted with a wide chemical range of debris from gravitationally torn apart planets.
The team's simulations produce a variety of planetary compositions, depending in part where the planet forms inside the dusty element-rich disk encircling a newborn star.
The scientists say that there are two key chemical ratios that determine the composition of terrestrial planets. And this could have far reaching implications for habitability, they caution.
One is the ratio of carbon to oxygen. Earth, Mars, and Venus all have more oxygen than carbon. But there could be "carbon planets" where the ratio tips in favor of more carbon. These planets would have bone dry surfaces where carbides would chemically break apart water to create carbon monoxide and methane rain. Living on such a planet would be like living in Los Angeles, with nothing but smog and asphalt.
The second factor is the magnesium to silicon ratio. Earth has nearly even amounts of both elements, but with a slight excess of silicon. Planets with significantly more silicon could have radically different kinds of plate tectonics and volcanic activity, say the researchers. Likewise, those atmospheres would be markedly different than Earth's.
The authors conclude: "There could be billions of Earth-like planets in the universe but a great majority of them may have a totally different internal and atmospheric structure. Building planets in chemically non-solar environments (which are very common in the universe) may lead to the formation of strange worlds, very different from the Earth!"
Does this really mean that a large fraction of planets we find in the habitable zones of stars might turn out to be fundamentally incompatible with life?
My guess is that life is capable of far more diversity and adapatation than we can imagine. Self-replicating matter might evolve to feel right at home on some of these exotic worlds. However, recognizing such bizarre life forms would probably be impossible without directly visiting the planets where they live.
"On carbon planets there might be creatures that eat silicates or oxides, and use carbon instead of oxygen for metabolism," says Marc Kuchner of NASA's Goddard Space Flight Center.
This research underscores that we need to be very cautions in tallying up what would be a bona fide Earth-like planet. Coming up with a realistic estimate might be decades away when we have large enough space telescopes to do a full chemical analysis. And, eventually vastly larger telescopes to do directing imaging of planetary geology.