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05/02/2007
Habitable? Possibly
I’m praying for a drought! For the second time in two weeks,
torrential rains resulted in an overabundance of water in our
basement/rec room. At least last week’s flooding was only about
an inch deep and we had already thrown out or removed virtually
everything that had been on the floor in our previous episode.
Water is also involved, if only by inference, in the biggest story
of the past week. Indeed, it could in the future turn out to be the
biggest story of the century – the discovery of a planet outside
our solar system that shows signs of life. We’re not there yet but
the discovery announced last week that made headlines in the
media is of the smallest planet outside our solar system in a
“habitable zone” as it orbits its star. Our Editor, Brian
Trumbore, mentions the planet in his Week in /Review column
but I hope to delve a bit deeper into the matter.
I first learned of the discovery of the new planet in an article by
Robert Boyd in the April 25 Star-Ledger and on Brian Williams’
NBC nightly news program. While the news accounts of the
discovery did a good job of reporting the findings, I wanted to
get a better idea of how the researchers came to the conclusion
that a possible earth-like planet orbits the star designated as
Gliese 581. I also wondered how certain they are of some of the
conclusions reported in the media. A visit to the Web site of the
European Southern Observatory (ESO) led me to the article,
actually a Letter to the Editor submitted to the journal
Astronomy and Astrophysics by Stephane Udry and his 10
European co-authors.
The method they used to detect the planet is what we called the
“wobble” technique in earlier columns. A planet orbiting a star
will, because of the gravitational attraction between the two
bodies, push or pull the star very slightly closer to or farther from
Earth as it orbits. The result is very slight shifts in the observed
frequencies of the light emitted by the star as the planet orbits.
It’s akin to the Doppler effect when a moving train goes by
blowing its horn. You hear a shift in the sound wave frequencies
as the train goes by. The bigger the planet, the bigger the shifts
in the frequencies of the light.
The astronomical name for the wobble method is the Radial
Velocity technique and the instrument used by the ESO workers
is called HARPS (High Accuracy Radial Velocity for Planetary
Searcher). HARPS, located in Chile, is an extremely sensitive
spectrograph and can measure velocities with an astounding
precision of a meter a second, which translates into about 2 miles
per hour. In my younger days I could walk around 4 miles an
hour. To measure velocities to that precision for objects 20 light-
years (over a hundred trillion miles) away is astounding.
Two years ago, the ESO workers found a planet some 15 times
more massive than Earth orbiting Gliese 581. This speedster
whips completely around the star in only 5.4 days and is roughly
the size of Neptune. The ESO data at the time also suggested the
presence of another planet and further measurements have
confirmed the existence of this planet. The planet has a mass
that may be as small as only 5 times Earth’s mass and a diameter
about twice that of our planet. The planet, which the European
researchers call “super-Earth”, is farther away from Gliese 581
and zips all the way around Gliese 581 in 13 days. But there’s
more. The researchers’ measurements indicated a third planet
orbiting the star. This planet is bigger than super-Earth but not
by that much, being 8 times more massive than Earth. It’s
farther away from the star than the other two planets and orbits
Gliese 581 in about 84 days.
Let’s concentrate on super-Earth. It’s 14 times closer to its star
than we are to our star, the Sun. Being so close to its sun, you
might well think that super-Earth would be awfully hot and could
not harbor life as we know it. But let’s take a closer look at
Gliese 581, one of a hundred closest stars to our Sun. Of these
hundred closest stars, 80 of them are “red dwarfs”. Red dwarfs
are smaller than our Sun and are at least 50 times fainter than our
own star. Gliese 581 is a red dwarf, about a third the mass of our
Sun. Not being as bright as the Sun, you can snuggle up much
closer to 581 and still be in that magical “habitable zone” – a
region where the temperatures are such that liquid water can
exist. The astronomers calculate that super-Earth is at the
“warm” edge of the habitable zone and that the other smaller
planet is at the “cold” edge of the zone.
The researchers estimate that the mean temperature on super-
Earth lies somewhere in the range from 0 to 40 degrees Celsius
(32 to 104 degrees Fahrenheit), temperatures we experience here
in New Jersey. Their models also suggest that super-Earth is a
rocky planet or one covered with oceans. All these conclusions
are based on measurements that detect velocity differences of
only in the neighborhood of 5 miles per hour, a brisk walk. The
extraordinary precision of these HARPS measurements account
for the fact that, out of 13 known planets with masses less than
20 Earth masses outside our solar system, 11 were discovered by
workers using HARPS.
There are some caveats that should be noted. The masses
calculated from the velocity measurements are minimum masses.
Why? We don’t know the angles of the orbits of the three
planets as viewed from Earth. If a planet passes directly between
us and the center of the star, it will pull the star towards us more
strongly than if, in the worst case, it circles the star perpendicular
to us. (In that case, it wouldn’t pull the star towards us at all, or
extremely slightly at best.) Since the HARPS measurements
detect only the small changes in velocity as the planets orbit the
star, exact calculations of masses are possible only if we know
the angles of the orbits to us. Hopefully, future telescopes will
be powerful enough to actually see the planets orbiting. I’m
guessing that the fact that the ESO workers detect the shifts in
frequencies that they do gives them some confidence that the
orbits around Gliese 581 are more in line with us than
perpendicular to us.
The possibility that the third planet, the one near the cold edge of
the habitable zone, is an artifact is considered in the Letter to the
Editor. Apparently, there’s the possibility that dark spots on the
surface of the rotating Gliese 581, similar I imagine to the
sunspots on our Sun, could lead to erroneous conclusions from
the velocity measurements. However, the authors point out that
Gliese 581 is a very stable star and no large spots have been
found. They do say, however, that the long-term stability of the
star will have to be checked further.
The authors also caution that their estimates of temperatures on
super-Earth depend on the planet’s composition and the
composition and thickness of its atmosphere. Obviously, before
we can celebrate even the possibility of life on super-Earth,
there’s need for much more attention to super-Earth in future
ground- or space-based projects. Such projects will certainly be
aimed at studying the planet’s atmosphere for signs of CO2 or
other compounds signifying the possibility that the “habitable”
zone is truly inhabited by some form of life.
Finally, I was touched and delighted to see the videos on TV of
65-year-old Stephen Hawking floating in a plane engaged in
maneuvers simulating the weightlessness of space travel.
Hawking thinks that man will have to eventually leave Earth for
another habitat in order to survive and it seems that he wanted to
experience what it would feel like to be on that journey. Maybe
someday a colony of intrepid space travelers will head off to
super-Earth!
Allen F. Bortrum
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