Dec 05 2010
These are challenging scientific questions, but my real concern here is the pop culture use of the term “Earth-like,” especially by the media and bloggers, and its potential confusing effect on the public.
Late last month an international 10-person group of planet hunters led by Andrew Howard of UC Berkeley announced in Science (Vol. 330, 29 October, 2010) magazine that about 23% of Sun-like stars in our Galaxy probably have “Earth-mass” planets in close orbits (<50 day orbits). And they speculated that Earth-mass planets at 1 AU (one Earth-Sun distance in a 365 day orbit) should be even more abundant. Because it’s an exciting discovery it got major, well-deserved attention. The headlines included: “Galaxy Rich in Earth-Like Planets” … and CNN says “Galaxy may be full of ‘Earths,’ alien life” … “The galaxy (probably) abounds in Earth-like planets” … and the BBC quotes scientists who assert “there could be a billion Earth-like planets in our own galaxy…
While there is reason to believe that planets with some similarities to Earth may exist in the nearby Cosmos, the headlines are — shall we say — misleading.
This is important because, to the public, the two most enticing drivers of human expansion into the Cosmos during the approaching new International Space Age are: 1) Earth-like worlds, and 2) extraterrestrial life, especially with intelligence.
So it’s important to clarify what we know at this point, and most importantly, what is meant by an “Earth-like” planet.
Is One Earth-Mass Enough?
The answer is no. Just having one Earth mass does not make an Earth-like planet, because the Earth is a very complex, only party understood body, as we’ll sketch more below. To Howard et al.’s credit, in their article’s abstract, they begin by speaking of “how common Earth-like planets are” and subsequently refer to “close-in Earth-mass planet(s).” Really two quite different things. And fortunately, the expression “Earth-like” is not used again.
However, it’s not clear how many journalists or casual readers will notice the distinction, because it’s not the point of the article and is not explained.
The previous headlines suggest not many.
Is One Earth Mass at One AU Enough?
Again the answer is probably not, although it should give you surface liquid oceans and plate tectonics.
Ten years ago Peter Ward and Donald Brownlee explained why plate tectonics is essential to habitability in Rare Earth: Why Complex Life is Uncommon in the Universe. In addition to promoting global “environmental complexity,” subduction-related volcanism recycles CO2 back into the atmosphere maintaining above-freezing surface temperatures so abundant liquid water can exist.
But even plate tectonics probably isn’t enough to guarantee Earth-like.
For example, the Moon acts as an essential gravitational anchor to Earth’s axial tilt. Without our Moon, the axial tilt would experience chaotic swings between 0 and almost 90 degrees — as the other terrestrial planets apparently have — which would cause extreme climate variations.
We owe our present climate stability to an exceptional event: the presence of the Moon.
Climate stability and habitability are also influenced by impact rates throughout Earth’s history, and Jupiter — with its 300+ Earth masses — has played a major role. Recent results of Horner and Jones (International Journal of Astrobiology, Vol. 9) have confirmed earlier results from the 1990s that Jupiter significantly reduces the Earth impact probability of Oort Cloud comets, although its effects on asteroids and short-period comets appear mixed.
What Can We Realistically Say Now About Earth-Like Planets in the Galaxy?
According to NASA’s numbers, “smaller planets outnumber larger ones.” But their data only goes down to 3 Earth masses; anything smaller than that can’t yet be detected. So they have to do a mass extrapolation, and that’s where the 23% number of Sun-like stars with Earth-mass planets in close orbits (<50 days) comes from. However, their extrapolation is based on a 2008 study of Jupiter-style planets around Sun-like stars that they apply to much smaller Earth-like planets, which they admit “probably differ.”
But Earth-mass planets in close orbits (well inside Mercury) cannot be Earth-like, so we need another extrapolation — this time based on distance. Using the same 2008 study for Jupiter-style planets we can also estimate the occurrence of Earth-mass planets at 1 AU: I get 38% of Sun-like stars with Earth-mass planets at 1 AU, although the fraction could be larger.
While both extrapolations are uncertain, the second one was mentioned only speculatively in the Howard et al. article — although “billions” came out in the media. Caution is also suggested by their admission that planetary formation models predict a “planet desert” in close orbits due to rapid inward migration of Jupiter-size planets and their gravitational interactions with inner planets. While the planet desert is inconsistent with close orbit planets, it may be correct farther out near 1 AU. This is still an area of active research.
A Few Summary Questions:
1) Are there billions of Earth-like planets in our Galaxy where you could live like you do here? Currently, given the complexity of Earth systems, we have no reason to believe that; all we know of is one.
2) Are there billions of Earth-mass planets in close orbits (or at 1 AU) around Sun-like stars in the Galaxy?
To the ebullient planet hunters the answer is ‘Yes’. But this is clearly an opinion that awaits observational confirmation.
3) Are lots of Earth-mass (but not Earth-like) worlds, even without complex forms of life, still good news for us? Obviously yes, as we begin the colonization of space during the coming Maslow Window. It’s an ebullient, Star Trek-like vision to imagine numerous, untouched, Earth-mass worlds — some even at the right stellar distance — patiently awaiting us later in this century and beyond.
In reality, large numbers of Earth-mass planets, but only a few truly Earth-like worlds, are what we would expect from Ward and Brownlee’s Rare Earth Hypothesis. And it would explain our preliminary observation that “complex life is uncommon in the Universe.”