Public and
scientific media recently headlined spectacular claims of discovery of
billions of other Earths, some “just around the corner.” But is this
hype, hope, or reality?
The spectacular claims include:
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Astronomers found 647 Earth-like planets
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25 Billion habitable planets exist in our galaxy
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Water was found on five exoplanets
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Earth 2.0 may be only 12 light-years away
Some
context is important. In May, the $550 million Kepler Space Telescope,
which was marginally able to detect Earth-like planets in the habitable
zones of Sun-like stars, failed before gathering definitive data.
Scientists now want a more capable, and more expensive, new satellite.
Is funding more likely if habitable planets are extremely rare or “just
around the corner?” In Big Science as elsewhere, sizzle sells.
To
understand what’s real, I read the original scientific publications in
the Proceedings of the National Academy of Science (PNAS), the Monthly
Notices of the Royal Astronomical Society, and the Astrophysical
Journal. I also analyzed data on Kepler’s website and NASA/Caltech’s
archive of exoplanets “officially confirmed” as of Dec. 1, 2013.
My conclusions are:
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Planets may be as numerous as stars.
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Recent claims are more spectacular than the evidence.
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What some call Earth-like is far from habitable.
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Earth is remarkably rare, but probably not unique.
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We are nowhere near finding a true Earth 2.0.
While
Scientific American;s website said Kepler found 647 possible Earths,
Kepler’s website actually lists only 167 confirmed planets of all types.
Only 13 are within 30% of Earth’s size and all those have surface
temperatures above 260°F. Anyone who calls that Earth-like should be
exiled there.
The Hubble
Space Telescope did find water vapor traces on five exoplanets. All
these were “hot Jupiters” roasting in 400 to 2500 times Earth’s
sunlight.
The key
paper was by Erik Petigura et. al. in PNAS. Given Kepler’s meager data,
this is fine research, but certainly open to scientific challenge. They
state: “Detections of Earth-sized planets in [Earth-sized] orbits are
expected to be rare in this study…Indeed, we did not detect any
such planet…” (My emphasis) Without seeing any, how can they claim
billions exist? Never underestimate clever people with powerful
computers and agendas.
In brief,
they define Earth-like far too broadly, make bold swags, and extrapolate
liberally. (Swag is a technical term for Scientific Wild A** Guess.)
Extrapolation
is the art of estimating the unknown from the known. For example,
imagine having survival data for Americans, but only for those under 50
years old. The dark blue line in the graph below shows the fraction of
Americans still alive at each age up to 50. To estimate our life
expectancy with only this limited data, we could assume the trend from
15 to 49 continues indefinitely. The red band extrapolates that trend,
and shows that 77% of us will still be alive at age 200. How wonderful!
The nearest bicentennial man must be “just around the corner.” Alas,
reality is the light blue line.
Clearly,
extrapolating can yield false conclusions. It can work if one
understands the underlying physics and uses an appropriate mathematical
model. We do not, however, have an adequate model of planet formation
and abundance — indeed our ignorance is why we launched Kepler in the
first place.
Petigura
separates detected exoplanets into “bins”, each spanning a range of
planet radius, orbital period, and intensity of host star’s light
hitting the planet. For each bin, Petigura computes the odds of
detecting a planet with that bin’s parameters. With modern computing
power, binning is a crude approach; one should compute odds for each
detected planet instead of averaging over very broad ranges.
For the
“Earth” bin, the computed detection odds are 1 in 585. For each detected
exoplanet in that bin, Petigura claims 585 actually exist, a huge
extrapolation that may well be far off. The detection odds depend
critically on the radii of the star and the exoplanet’s orbit, neither
of which is actually measured. Kepler doesn’t take “pictures” of stars
or exoplanets; it only measures dips in starlight intensity, which
astronomers interpret as “transits”, exoplanets passing in front of a
star and blocking a tiny portion of its light. The most important
quantities, planet size and orbit, are estimated.
Petigura
detected 8 exoplanets in Earth’s bin and claimed 4673 others exist but
were missed, and thus “22% ± 8% of Sun-like stars” have planets like
Earth.
Petigura
says Kepler’s data is “challenged by correlated, non-stationary,
non-Gaussian noise.” While scientists understand and can model simple
noise, one must wonder if Petigura can properly model this more complex,
ill-defined noise. They also say 28% of their initial exoplanet
candidates were actually eclipsing binary stars. It’s normally easy to
distinguish stars from planets. This 28% error rate shows how
challenging and uncertain their analysis is.
Petigura
defines the “Earth” bin as planets whose radii are 100% to 200% of
Earth’s and whose starlight intensities are 100% to 400% of Earth’s.
Many fear the dire consequences of a 1% rise (3°C) in Earth’s
temperature — imagine 300% more sunlight.
Reducing
the upper limit on starlight from 400% to 200% drops the number of
Earth-like exoplanets from 8 to 2, and their abundance from 22% to 5.5%.
Reducing that limit to 186% leaves only 1 Earth-like exoplanet and the
extrapolated abundance drops below 3%. It’s disturbing that the final
result depends so critically on a questionable definition.
Petigura’s
Earth bin is far too broad. The NASA/Caltech archive, the field’s gold
standard, lists 935 confirmed exoplanets from all sources including
Kepler. Of the 296 exoplanets with measured masses and radii, 19 are
within 100% to 200% of Earth’s radius. Only 2 of those are less massive
than Neptune, an uninhabitable gas giant. Thus 90% of exoplanets in
Petigura’s radius range are not Earth-like based on real unbiased,
unextrapolated data.
It’s harder
to set a firm starlight intensity limit. Here are some choices. Of 935
confirmed exoplanets, 365 have known starlight intensity. Comparing with
Earth: only 1 is within ±10%, 5 are within ±20%, and 16 are within a
factor of 2.
Furthermore,
all the above only addresses two planetary characteristics: size and
surface temperature. In my books, I list 14 remarkably special
characteristics of Earth. For each characteristic currently quantifiable
with real data, Earth is better than 90% to 99.5% of all known planets.
If the odds of Earth 2.0 were 10% raised to the 14th power, there would
be zero other habitable planets in our galaxy.
Without
definitive data, all we really know is that the number of habitable
planets in our galaxy is between zero and 25 billion.
‘Tis the season to rejoice that we’re on the only known habitable planet.
Very best wishes for the Holidays and a Happy New Year,
Robert
December 17th, 2013
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