Is Planet Gliese 581g, really “the first
potentially habitable Alien World?
by Mike Wall, SPACE.com Senior Writer
Date: 23 July 2012 Time: 02:13 PM ET
This
artist's conception shows the inner four planets of the Gliese 581 system and
their host star. The large planet in the foreground is Gliese 581g,
which is in the middle of the star's habitable zone and is only two to three
times as massive as Earth. Some researchers aren't convinced Gliese 581g
exists, however.
CREDIT: Lynette Cook
Nearly
two years after spotting Gliese 581g, the celebrated "first potentially
habitable" alien world, the planet's discoverers continue to fight for its
existence.
The
discovery of Gliese 581g made headlines around the world in September 2010,
because the planet was said to orbit in the middle of its star's
"habitable zone" — that just-right range of distances where liquid
water, and perhaps life as we know it, could exist.
Just
a few weeks later, however, another prominent research team began casting doubt
on the find, saying the alien planet didn't show up in their observations. This group, led by Michel
Mayor of the Geneva Observatory in Switzerland, had found the previously known
four planets in the Gliese 581 system.
But in a new study that will be published Aug. 1, 581g's discoverers examine the Swiss team's since-expanded data set and take issue with their conclusions, saying that the evidence supports the planet's existence after all.
But in a new study that will be published Aug. 1, 581g's discoverers examine the Swiss team's since-expanded data set and take issue with their conclusions, saying that the evidence supports the planet's existence after all.
The data
and analyses "point to there being at least one other planet beyond the
confirmed 4, a 5th planet, with a period in the 26-39-day regime," lead
author Steve Vogt, an astronomer at the University of California, Santa Cruz,
told SPACE.com via email. [The Strangest Alien Planets]
"This
5th planet would have a mass of only 2-3 [times that of] Earth, and would orbit
pretty much squarely in the star's habitable zone," he added.
The orbits
of planets in the Gliese 581 system are compared to those of our own solar
system. The Gliese 581 star has about 30 percent the mass of our sun, and the
outermost planet is closer to its star than we are to the sun. Gliese 581d
might be able to sustain liquid water on its surface.
CREDIT: Zina Deretsky, National Science
Foundation.
The saga of
Gliese 581g
The Gliese
581 system is a near neighbor of Earth, located just 20 light-years away. On
Sept. 29, 2010, Vogt and his team announced that they'd found two new planets
in the system, bringing its number of known worlds to six.
One of the
newfound planets, 581f, sits far from the host star, completing one orbit every
400 days or so. But 581g got far more attention, because it was said to be the
first rocky, roughly Earth-size alien planet ever found in the habitable zone.
Vogt and
his team detected the two planets using the radial velocity — or Doppler —
method, which looks for tiny wobbles in a star's movement caused by the
gravitational tugs of orbiting planets. They studied data from two different
instruments: the HARPS spectrograph, on a telescope in Chile, and the HIRES
spectrograph, on Hawaii's Keck Telescope.
At an
astronomy conference in October 2010, however, the Swiss team announced that
they couldn't find 581g or 581f in an expanded set of HARPS data. Over the next
few months, several other research groups also disputed the two planets' existence
after re-analyzing available HARPS and HIRES observations using different
statistical methods than those employed by Vogt and his colleagues.
And in
September 2011, the Swiss team completed a study that looked at even more HARPS
observations — a total of 240 radial-velocity data points. Their manuscript was
submitted to the journal Astronomy & Astrophysics and made available on the
astronomy preprint website arxiv.org.
The Swiss
group's conclusions were much the same in the more recent paper as they had
been before .
"Our
dataset therefore has strong diagnostic power for planets with the parameters
of Gl 581f and Gl 581g, and we conclude that the Gl 581 system is unlikely to
contain planets with those characteristics," the researchers write in the
paper. [Gallery: The Smallest Alien Planets]
Doubting
the doubters
The Swiss
team made their 240 HARPS data points available, and Vogt and his colleagues
decided to do their own analysis, which forms the core of their new paper.
They came
to quite different conclusions, with Vogt's team finding that the data support
the existence of 581g as long as the system's planets are all in roughly
circular orbits. Making this assumption is not a stretch, Vogt said.
"I do
believe that the all-circular-orbits solution is the most defensible and
credible," he said. "For all the reasons I explain in detail [in the
paper], it wins on account of dynamic stability, goodness-of-fit, and principle
of parsimomy (Occam's Razor)."
Gliese 581f
doesn't show up in the new analysis performed by Vogt and his colleagues, but
the scientists say that doesn't necessarily mean the planet is dead.
"That
does not mean it might not be there in a much larger combined data set. The
present paper only is dealing with that 240-point HARPS data set, all by its
lonesome," Vogt said. "We do have all of our previous data, plus new
data trickling in slowly."
The study
also argues that 581g's existence is not incompatible with the findings of the
doubting statistical papers, which the discovery team said employed an
extremely conservative detection threshold.
http://www.space.com/10751-kepler-reveals-amazing-amount-planets-habitable.html
Battling it
out
Vogt also
said that his team could not reproduce the Swiss researchers' 2011 results
without tossing out a handful of data points.
"Our
calculations of these quantities do not match those presented by the Swiss.
Therefore, unless we are doing it wrong, we are obliged to conclude that they
omitted points," Vogt said. "We were easily able to recover their
values when we omitted 5 points."
"I
don't know whether this omission was intentional or a mistake," he added.
"I can only say that, if it was a mistake, they've been making that same
mistake more than once now, not only in this paper, but in other papers as
well."
The study
led by Vogt is now available on arxiv.org, and it's slated to come out Aug. 1
in the journal Astronomische Nachrichten ("Astronomical Notes"),
which is based in Germany. But Vogt said it was first submitted to, and
accepted by, The Astrophysical Journal.
The
Astrophysical Journal wouldn't publish the study, Vogt said, until the Swiss
team's paper was formally accepted by Astronomy & Astrophysics. When this
still hadn't happened after nine months, Vogt and his colleagues decided to
pull the paper and publish in the German journal.
Vogt said
the Swiss group's manuscript still hasn't been accepted.
Many
habitable planets out there
It will
likely take more observations to sort out for sure how many planets circle the
star Gliese 581, Vogt said. But the exoplanet community isn't exactly fixated
on the issue.
Planet
hunters are finding more and more worlds beyond our solar system. The tally is
nearing 800 by some counts, with thousands more awaiting confirmation by
follow-up observations. And while researchers have yet to find a true alien
Earth, they're getting closer and closer.
This past
December, for example, NASA's Kepler space telescope confirmed its first planet
in the habitable zone, a "super Earth" known as Kepler-22b that's
thought to be 2.4 times as wide as our planet. And Vogt is part of a team that
announced another potentially habitable super Earth, known as Gliese 667Cc, in
February.
"The
take-away message is, regardless of who eventually gets bragging rights to the
discovery of the first truly confirmed habitable Earth-sized planet, we are
starting to find them in unexpected numbers, and unexpectedly nearby,"
Vogt said. "That means there are many, many out there, at least tens of
billions or more, in our own galaxy alone."
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System is a very important world into IRIS book of Alfonso Sánchez Ortega.
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