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07/06/2005
A Huge Neighbor and a Small Planet
Astronomy and outer space offer no end of exciting events and
discoveries these days. This week it was the remarkable success
of Deep Impact, the probe that smashed into the comet Tempel 1
on July 4. This was one NASA mission that was flawless in its
execution. The main objective of the mission was to expose the
comet’s pristine composition, on the assumption that Tempel’s
contents are the same materials that went into making up our
solar system billions of years ago when it was being formed.
Coincidentally, I had planned to write this week about how solar
systems are formed. However, with the possibility that some
relevant results from Deep Impact might be available shortly, I’ll
postpone that topic until next week. Besides, there’s other
spectacular news on a new planet in another solar system and on
the well-known Andromeda galaxy, which has suddenly become
vastly larger than thought heretofore. Andromeda is the nearest
major galaxy to our own Milky Way and is said to be visible to
the naked eye as a fuzzy patch on a crystal clear night. (There
are very few such nights in our area.) Andromeda is also the
galaxy that’s headed our way in what should be a doozy of a
deep impact!
Andromeda is a classic spiral galaxy and is usually the one
shown in textbooks or encyclopedias. It’s a nice disc-shaped
galaxy and its disc of stars and visible matter is roughly 100,000
light years (about 600,000,000,000,000,000 miles) in diameter.
At least that was the general supposition until a meeting a few
weeks ago of the American Astronomical Society in
Minneapolis. An article on the meeting by Robert Irion in the
June 17 issue of Science cites work reported by Nial Tanvir of
the University of Hertfordshire in the UK and Rodrigo Ibata of
the Strasbourg Observatory in France and their colleagues. They
used the Isaac Newton Telescope on the Canary Islands to search
the regions surrounding Andromeda’s main spiral disc. They
were surprised to find swarms of faint stars in these outlying
regions. These stars lie in what was considered to be a “halo” of
stars from other disrupted galaxies that had their own deep
impacts with the voracious Andromeda.
It was expected that these stars would be flitting about in a
random manner and that they were not in the mainstream of the
stars circling the core of Andromeda. However, when the Keck
II telescope on Mauna Kea in Hawaii was brought into the
picture by Scott Chapman and coworkers, they looked at about
5,000 stars and found that many revolved around smoothly as a
huge extension of the disk we knew and loved. If you looked up
in the sky with a telescope that could reveal these outer stars, you
would see that Andromeda covers an amazing space equal to the
width of a dozen full moons lined up in a row! Andromeda is
three times bigger than we thought previously.
But wait, there may be more. Puragra Guhathakurta of Cal Tech
and coworkers now claim to have found stars in Andromeda’s
halo some 500,000 light years from its center. That would make
Andromeda a million light years in diameter and its outer reaches
would be a fifth of the distance to our Milky Way. That deep
impact may come sooner than we thought!
Last week I noted that Jack Kilby received the Nobel Prize for
his invention of the integrated circuit. I maintain that another
invention, the CCD (charge-coupled device) invented by Willard
Boyle and George Smith at Bell Labs, should be similarly
acknowledged. Aside from its role in cameras, camcorders and
the like, the CCD’s capability of detecting photons has opened
up vast new worlds in astronomy, perhaps even the above
Andromeda story. It certainly played a role in the recently
headlined discovery of a new planet outside our solar system.
Finding these so-called extrasolar planets is by now old hat, with
roughly 150 now on the books. We’ve talked before about how
astronomers have detected these planets in most cases by looking
at the “wobble” in a star’s light as the planet pushes and pulls the
star slightly out of position as the planet orbits the star. A prime
goal, of course, is to find another planet that resembles Earth
sufficiently that it could harbor life. We’re getting closer.
About 8 years ago, Paul Butler of the Carnegie Institution in
Washington, and Geoffrey Marcy of the University of California,
Berkeley, were looking at a “wobbling” small star, Gliese 876, a
red dwarf about a third the size of our Sun. In 1998, they
reported finding a planet orbiting 876 that was about twice the
size of Jupiter. They and their fellow researchers didn’t abandon
876 and in 2001 they found another planet about half the size of
Jupiter.
As they gathered more data, the researchers found that the two
planets were influencing each other’s motions such that they
were in resonance. These two giant gas planets gravitational
pulls on each other are akin to a dance, with the outer planet
taking 60 days to orbit 876, the inner one just half as long, 30
days. If you’re a smart theorist, you can model the motion of a
resonant duo and predict how the wobble of 876’s light should
behave with time.
But there was a problem. When theorists Eugenio Rivera and
Jack Lissauer plugged the numbers into their model for the two
planets orbiting 876, there was a slight discrepancy that indicated
a possible third planet. They needed more precise data. Last
August, Steven Vogt, who designed the high resolution
spectrometer used in the Keck telescope on Mauna Kea, worked
with his Santa Cruz team to upgrade the spectrometer’s CCD
detectors. Last month, the 876 team held a press conference at
the National Science Foundation in Arlington, Virginia.
The upgraded CCD detectors provided the more precise data
needed to confirm that there was indeed another planet. This
planet is likely only 7 or 8 times the mass of Earth and about half
the size of the smallest other extrasolar planet detected to date.
Until now, the planets that have been detected outside our solar
system have been large gaseous planets.
Unfortunately, the new small planet orbits very close to 876,
only about two millions miles away and the temperature is
probably 400 to 750 degrees Fahrenheit. At a minimum, it’s
about the same temperature you’d set your oven to cook a
tenderloin of beef. It’s a short year on the small planet – just a
tad less than 2 full days! Not much time to adjust one’s
wardrobe for the changing seasons! The composition of the
planet is not pinned down with certainty but the researchers
believe that it’s a rocky planet. However, there is a chance that
it’s more like a smaller version of Neptune, with a thick
atmosphere of hydrogen, water and other gases.
With the new upgraded CCD detectors, the hunt is on for planets
around other red dwarfs such as Gliese 876. These red dwarfs
put out less energy than our Sun and a small planet out far
enough for a 10 or 20-day year might be cool enough for liquid
water to exist. Maybe we’ll find a hospitable planet to colonize
before Andromeda comes calling!
Allen F. Bortrum
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