Throughout 2011 there was a string of breathless news stories about astronomers finding extrasolar planets in the habitable zones surrounding their stars.
This is the “Goldilocks Zone” where temperatures are just right for water to remain in liquid form and presumably nurture life as we know it.
Last week, NASA announced the discovery of the first two Earth-sized planets orbiting a sun-like star. That followed an announcement from the Kepler space observatory two weeks before that scientists had discovered a planet roughly twice the size of Earth orbiting inside its star’s habitable zone.
SETI astronomers are firing up their Allen Array radio telescope to check these worlds for signs of intelligent life.
But using the term Earth-like is a stretch at best, and misleading at worst.
We don’t have a clue about the physical nature or processes on these worlds any more than an air traffic controller’s radar blip tells him what meals are being served on a commercial flight.
Saying that liquid water could exist is OK, but to imply it does exist in the phrase “Earth-like planet,” is very presumptive. Even press release artistic illustrations “lead the witness” by showing idyllic water-drenched worlds.
The bottom line is that we don’t know how Earth got tanked-up with its water supply. So how might we begin to guess what’s happening on worlds thousands of light-years away?
“If we need exotic mechanisms to get water onto Earth, then maybe it suggests life is not prevalent in these exoplanetary systems,” astrobiologist Karen Meech of the University of Hawaii recently told astronomers in a colloquium at the Space Telescope Science Institute.
The oceans account for merely one-quarter of one percent of Earth’s mass. Another one-tenth of a percent may be in Earth’s mantle. But if we could probe deeper, down into the core, Earth could conceivably have 50 oceans worth of water locked away from the days of our planet’s formation. (This is somewhat bemusing considering that Jules Verne wrote about a great subterranean ocean in the 1864 “A Journey to the Center of the Earth.”)
With water potentially so locked away, “we may never know how much water Earth really has,” says Meech.
This complicates several competing theories for how Earth got its water supply in the first place. We know water is everywhere in the solar system, especially among the planets and moons of the outer solar system. They lie beyond the “frost line” (roughly the distance of the asteroid belt) where water can remain a solid. By comparison the baked rocky planets Mercury and Venus seem bone dry, and Mars looks arid at best.
From the geologic record we do know that oceans were here on Earth just a few hundred million years after our planet’s formation 4.6 billion years ago.
The Spitzer and Herschel space telescope observations of the young star TW Hydrae (seen the Hubble Space Telescope infrared picture below) shows that its protoplanetary disk contains enough water molecules to make 6,000 oceans (should we rename it TW Hydro?). The star also has a so-called snowline beyond a range of a few hundred million miles, the same as with our own solar system.
Dust grains in the sun’s protostellar nebula were likely porous and could have captured water molecules for the newly forming Earth. But given the violent birth of Earth though accretion and differentiation, could they have remained intact?
Another possibility is that the young Earth manufactured its own water. The early Earth was so hot it had an ocean of molten magma. The oxygen in that magma could have combined with hydrogen in protostellar gas envelope, before it was dissipated away by the glare of the newborn sun.
If water was instead imported from comets and asteroids, Meech estimates would take 20 million medium sized comets to fill Earth’s oceans. but she thinks that only 1/50th the ocean’s volume came from comets.
Recent computer simulations show that, dynamically, all hell broke loose in the solar system if the outer planets migrated in their orbits — a phenomenon commonly seen in exoplanetary systems. Our young world would have been pelted with water-bearing asteroids that were thrown into Earth-crossing elliptical orbits.
This would explain the late heavy bombardment at roughly 4 billion years ago as recorded on the moon and other solar system bodies. On the other hand, was water transported to the early Earth by a class of objects that no longer exist? And, did the water appear late, early, or in sporadic episodes in Earth’s formative years?
The picture is so complicated that it’s safest to say that water probably came to Earth from many sources: comets, hydrated asteroids, solar nebula gasses, and chemical processing on Earth’s surface. This makes the question of trying to determine the extraterrestrial source of water from its chemical fingerprint, called the deuterium/hydrogen ratio, a moot point says Meech.
The origin of Earth’s water is such a complex question that it requires interdisciplinary groups of scientists to try and answer how much water Earth contains, when did it arrive, and where did it come from?
Because we don’t even know how much water Earth has, we don’t know if we live on a comparatively dry or wet planet. At least we know there was enough water to jump-start and maintain plate tectonics, which relies on water as much as a car engine relies on motor oil. Plate tectonics really is the essence of what makes our planet “Earth-like.” But how much more or less water would make us not “Earth-like?”
Finally, how is water manufactured and distributed in exoplanetary systems? This is gaping hole in our knowledge that needs to be filled before yet another so-called “Earth-like planet” is announced to the world.
Image credits: NASA, NASA/JSC Gateway to Astronaut Photography of Earth.