|
12/04/2001
Something Special in the Air
I''ve just returned from my 3-mile walk on December 1st and the
temperature is 70 degrees. Our neighbors are in shorts and T-
shirts while walking or raking their leaves. We have just
finished one of the warmest Novembers on record here in this
part of New Jersey, some 20 miles from Ground Zero. In New
York, they''re concerned about possible pollutants in the air from
the still-smoking debris of the World Trade Center. More
broadly, there''s some concern that the unusually warm weather
might be a result of global warming, spurred in part by the
increasing amount of carbon dioxide in the atmosphere.
What about the atmospheres of planets other than our own? In
yesterday''s Star Ledger, there was an article by Paul Recer that
discussed a report in the current issue of Science on the presence
of hydrogen in the atmosphere of Mars. I haven''t received my
own copy of this issue of Science but will assume that the major
conclusions in Recer''s article are correct. The main finding is
that there are hydrogen molecules, H2, floating around in the
atmosphere surrounding Mars. The presence of molecular
hydrogen is taken as confirmation that Mars once had a heck of a
lot of water. In fact, the article says that Mars once had more
water per square mile than we have on earth today. So much
water that it would have covered Mars to a depth of nearly a
mile! Of course, all that water leads to speculation about the
possibility of life on Mars at one time. Did you see reports of
that fossil found in Africa of what apparently was a 40-foot long
crocodile? Wouldn''t it be neat if its ancestor came from an ocean
on Mars?
I must admit that I''m unclear as to how they can deduce the
presence of a huge amount of water in Mars'' past based on
hydrogen molecules in the atmosphere today. The article implies
that the hydrogen molecules come from the breakdown of water
into hydrogen and oxygen. Vladimir Krasnopolsky of Catholic
University and Paul Feldman of Johns Hopkins reportedly
gathered their data on hydrogen from the far Ultraviolet
Spectroscopic Explorer spacecraft.
Spectroscopy is a time-honored tool for gathering information on
what''s out there in our universe. How is this possible? Let''s say
that we want to detect sodium in the vicinity of a distant star. A
sodium atom has electrons orbiting around a nucleus. Let''s focus
on just one of the electrons. If our sodium atom gets heated up,
like in or around a hot star, the electron can get kicked out of its
normal orbit to a higher one. Only certain orbits are possible.
It''s sort of like kicking the planet Mercury out to Venus'' orbit.
When the electron drops back to its normal orbit, a photon of
light is emitted corresponding to the energy difference between
the two orbits. The light''s color depends on this energy and can
be visible light or invisible ultraviolet or infrared light.
When I was in college, one test for sodium was to take a sodium
compound such as sodium chloride, table salt, put some on a
platinum wire and stick it in the flame of a Bunsen burner. A
bright yellow color results. With lithium chloride, a reddish
color is seen. Each element has its own sets of energy levels and
spectra, patterns of light emission, when heated. So, if we want
to detect sodium, we tune our spectroscopic instruments to detect
the "colors" of light that result from electrons hopping among the
various orbits of the sodium atom.
In fact, let''s tune the instruments on the Hubble Space Telescope
to detect sodium and point our telescope at HD 209458. What is
HD 209458? It''s a star like our sun some 150 light years away in
the constellation Pegasus. It''s not a very bright star but is bright
enough that if you had just an amateur''s type of telescope you
might be able to spot it. Sure enough, when we look at the star
with the Hubble we see spectra that indicate the presence of
sodium. This doesn''t sound particularly exciting, does it?
What''s exciting is that HD 209458 has a planet orbiting it. This
planet is one of the many planets that have been found orbiting
distant stars over the past decade or so. In an earlier column, we
discussed how none of these planets has actually been seen.
Virtually all of them have been detected by looking at the
"wobble" in the star''s motion caused by the gravity of the planet
pushing or pulling the star, if ever so slightly.
HD 209458 is the exception. It''s planet orbits in a path that
fortuitously lines up edge-on facing the earth. In other words, if
we could see the planet, as it circles around the star we would see
the planet cross the center of the face of the star and then
disappear behind it. I believe I may actually have mentioned
earlier that this lucky positioning of the planet''s orbit allowed the
planet to be detected directly. By directly, I mean that
astronomers using the Hubble measured the slight dimming of
the light from the star caused by the planet blocking the starlight
as it crosses between the star and the earth.
To actually confirm a planet''s existence that way was exciting
enough, but the latest results are even more momentous. To set
the stage, we''ve shown that HD 209458 emits photons of light
from its sodium. Suppose those photons sail along in space and
hit some other sodium atoms. If a photon of that color hits
another sodium atom just right, it has just the energy needed to
kick an electron in the sodium atom up to a higher orbit. If so,
that photon will disappear and the intervening sodium will have
absorbed some of the sodium light from the star.
Now here''s the latest exciting news. Let''s point our Hubble
telescope at HD 209458 and make super precise measurements
of the sodium spectra as the planet crosses in front of the star.
By following until the planet just slips behind the star, we can
detect changes in the sodium spectra. We see some changes that
indicate absorption of the sodium light from the star. Also,
there''s some scattering of that light.
Put this all together and we have a monumental achievement.
We have demonstrated for the first time that a planet outside our
solar system not only has an atmosphere, but also we''ve begun to
find out what''s in that atmosphere. Ok, we didn''t do this
ourselves. Two investigators named David Charbonneau and
Timothy Brown are credited with the discovery. They are now
tuning the Hubble to detect what other elements are in the
planet''s atmosphere.
A trip to the University of California Planet Search Web site
shows that as of November 19, 2001 some 76 planets have been
found orbiting distant stars. The masses of these planets range
from 17 times the mass of Jupiter to only 17 percent of a Jupiter.
Our planet orbiting HD 209458 is a gaseous planet about 63
percent of a Jupiter. The HD 209458 planet is only 4 million
miles from the star, closer than Mercury is to our sun. Being so
close, the planet is also zipping right along, orbiting the star once
every 3.5 days. That is, a year is a hundred times shorter than
our year on earth. If you lived there, think how fast you''d age!
But this short year is great for astronomers. They only have to
wait a couple days and the planet is back in sight. Well, not
exactly in sight. As we''ve noted, our telescopes aren''t powerful
enough yet to actually see even a Jupiter-size planet.
Unfortunately, HD 209458''s planet is not exactly a prime
candidate for life. Its closeness to the star gives it a toasty
temperature of around 2,000 degrees Fahrenheit! However, the
detection and analysis of its atmosphere is the first small step on
the long journey to the anticipated detection of earth-size planets
with atmospheres that may contain elements and compounds that
indicate the presence of life. Telescopes are under construction,
which hopefully will provide the power to detect and measure
such planets and their atmospheres. However, we''ll have to wait
another 8 years or so before they are in operation.
Stay tuned!
Allen F. Bortrum
|
