Artist’s
representation of the “habitable zone,” the range of orbits where
liquid water is permitted on the surface of a planet. The authors find
that 22±8% of Sun-like stars harbor a planet between one and two times
the size of Earth in the habitable zone. Credit: Petigura/UC Berkeley, Howard/UH-Manoa, Marcy/UC Berkeley. Larger image
Analysis
of four years of precision measurements from Kepler shows that 22±8% of
Sun-like stars have Earth-sized planets in the habitable zone. If these
planets are as prevalent locally as they are in Kepler field, then the
distance to the nearest one is around 12 light-years.zone. Credit: Petigura/UC Berkeley, Howard/UH-Manoa, Marcy/UC Berkeley. Larger image
Waimea, Hawaii – Scientists from University
of California, Berkeley, and University of Hawaii, Manoa, have
statistically determined
that twenty percent of Sun-like stars in our galaxy have Earth-sized
planets that
could host life. The findings, gleaned from data collected from NASA’s
Kepler
spacecraft and the W. M. Keck Observatory, now satisfy Kepler's primary
mission: to determine how many of the 100 billion stars in our galaxy
have
potentially habitable planets. The results are being published November 4
in the
journal Proceedings of the National
Academy of Sciences.
"What this means is,
when you look up at the thousands of stars in the night sky, the nearest
sun-like star with an Earth-size planet in its habitable zone is probably only
12 light years away and can be seen with the naked eye. That is amazing,"
said UC Berkeley graduate student Erik Petigura, who led the analysis of the
Kepler and Keck Observatory data.
"For NASA, this number – that every fifth star has a planet somewhat like Earth – is really important, because successor missions to Kepler will try to take an actual picture of a planet, and the size of the telescope they have to build depends on how close the nearest Earth-size planets are," said Andrew Howard, astronomer with the Institute for Astronomy at the University of Hawaii. "An abundance of planets orbiting nearby stars simplifies such follow-up missions."
"For NASA, this number – that every fifth star has a planet somewhat like Earth – is really important, because successor missions to Kepler will try to take an actual picture of a planet, and the size of the telescope they have to build depends on how close the nearest Earth-size planets are," said Andrew Howard, astronomer with the Institute for Astronomy at the University of Hawaii. "An abundance of planets orbiting nearby stars simplifies such follow-up missions."
Earth-size may not mean habitable
The team, which also included planet hunter Geoffrey Marcy, UC Berkeley professor of astronomy, cautioned that Earth-size planets in Earth-size orbits are not necessarily hospitable to life, even if they orbit in the habitable zone of a star where the temperature is not too hot and not too cold.
"Some may have thick atmospheres, making it so hot at the surface that DNA-like molecules would not survive. Others may have rocky surfaces that could harbor liquid water suitable for living organisms," Marcy said. "We don't know what range of planet types and their environments are suitable for life."
Just last week, Howard, Marcy and their colleagues provided hope that many such
planets actually are rocky. They reported that one Earth-sized planet discovered – albeit, a planet with a likely temperature of
2,000 Kelvin, which is far too hot for life as we know it – is the same density
as Earth and most likely composed of rock and iron, like Earth.
"This gives us some confidence that when we look out into the habitable zone, the planets Erik is describing may be Earth-size, rocky planets," Howard said.
Transiting planets
"This gives us some confidence that when we look out into the habitable zone, the planets Erik is describing may be Earth-size, rocky planets," Howard said.
Transiting planets
NASA launched the now crippled Kepler space telescope in 2009 to look for planets that cross in front of, or transit, their stars, which causes a slight diminution – about one hundredth of one percent – in the star's brightness. From among the 150,000 stars photographed every 30 minutes for four years, NASA's Kepler team reported more than 3,000 planet candidates. Many of these are much larger than Earth – ranging from large planets with thick atmospheres, like Neptune, to gas giants like Jupiter – or in orbits so close to their stars that they are roasted.
To sort them out, Petigura and his colleagues are using the twin,
10-meter
telescopes of the Keck Observatory on the summit of Mauna Kea, Hawaii to
obtain HIRES spectra of as many stars as possible. This will help them
determine each star's
true brightness and calculate the diameter of each transiting planet,
with an
emphasis on Earth-diameter planets.
HIRES (the High-Resolution
Echelle Spectrometer) produces spectra of single objects at very high
spectral resolution, yet covering a wide wavelength range. It does this
by separating the light into many "stripes" of spectra stacked across a
mosaic of three large CCD detectors. HIRES is famous for finding planets
orbiting other stars. Astronomers also use HIRES to study distant
galaxies and quasars, finding clues to the Big Bang.
The team focused on the 42,000 stars that are like the sun or slightly cooler and smaller, and found 603 candidate planets orbiting them. Only 10 of these were Earth-size, that is, one to two times the diameter of Earth and orbiting their star at a distance where they are heated to lukewarm temperatures suitable for life. The team's definition of habitable is that a planet receives between four times and one-quarter the amount of light that Earth receives from the sun.
A census of extrasolar planets
The team focused on the 42,000 stars that are like the sun or slightly cooler and smaller, and found 603 candidate planets orbiting them. Only 10 of these were Earth-size, that is, one to two times the diameter of Earth and orbiting their star at a distance where they are heated to lukewarm temperatures suitable for life. The team's definition of habitable is that a planet receives between four times and one-quarter the amount of light that Earth receives from the sun.
A census of extrasolar planets
The analysis subjected Petigura's planet-finding algorithms to a battery of tests in order to measure how many habitable zone, Earth-size planets they missed. Petigura actually introduced fake planets into the Kepler data in order to determine which ones his software could detect and which it couldn't.
"What we're doing is taking a census of extrasolar
planets, but we can't knock on every door. Only after injecting these fake
planets and measuring how many we actually found, could we really pin down the
number of real planets that we missed," Petigura said.
Accounting for missed planets, as well as the fact that only
a small fraction of planets are oriented so that they cross in front of their
host star as seen from Earth, allowed them to estimate that 22 percent of all
sun-like stars in the galaxy have Earth-size planets in their habitable zones.
"The primary goal of the Kepler mission was to answer
the question, When you look up in the night sky, what fraction of the stars
that you see have Earth-size planets at lukewarm temperatures so that water
would not be frozen into ice or vaporized into steam, but remain a liquid,
because liquid water is now understood to be the prerequisite for life,"
Marcy said. "Until now, no one knew exactly how common potentially
habitable planets were around Sun-like stars in the galaxy."
All of the potentially habitable planets found in their
survey are around K stars, which are cooler and slightly smaller than the sun,
Petigura said. But the team's analysis shows that the result for K stars can be
extrapolated to G stars like the sun. Had Kepler survived for an extended
mission, it would have obtained enough data to directly detect a handful of
Earth-size planets in the habitable zones of G-type stars.
If the stars in the Kepler field are representative of stars
in the solar neighborhood, then the nearest (Earth-size) planet is expected to
orbit a star that is less than 12 light-years from Earth and can be seen by the
unaided eye. Future instrumentation to image and take spectra of these Earths
need only observe a few dozen nearby stars to detect a sample of Earth-size
planets residing in the habitable zones of their host stars.
In January, the team reported a similar analysis of Kepler
data for scorched planets that orbit close to their stars. The new, more
complete analysis shows that "nature makes about as many planets in
hospitable orbits as in close-in orbits," Howard said.
The
W. M. Keck Observatory operates the largest, most scientifically
productive telescopes on Earth. The two, 10-meter optical/infrared
telescopes on the summit of Mauna Kea on the Island of Hawaii feature a
suite of advanced instruments including imagers, multi-object
spectrographs, high-resolution spectrographs, integral-field
spectroscopy and world-leading laser guide star adaptive optics systems.
The Observatory is a private 501(c) 3 non-profit organization and a
scientific partnership of the California Institute of Technology, the
University of California and NASA.
Science Contacts:
Erik Petigura
epetigura@berkeley.edu,
(650) 804-1379 (cell)
Andrew Howard
howard@ifa.hawaii.edu
(808) 208-1224 (cell)
Geoff Marcy
gmarcy@berkeley.edu,
(510) 759-9398 (cell)
Media Contact:
Steve Jefferson
sjefferson@keck.hawaii.edu
(808) 881-3827(cell)
