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20020703 Jupiter's Fraternal Twin (Donohue).doc
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From the Everett and Seattle Astronomical Societies, this is IT’S OVER
YOUR HEAD for the week beginning July 3rd, 2002, a look at what’s up in the sky
over Puget Sound.
In the wee hours of yesterday morning, July 2nd, the innermost planet
Mercury, and the second largest gas giant, beautiful ringed Saturn, appeared to
be only two-tenths of one degree apart from each other in the sky. As we
mentioned last week, they only appeared that way from Earth’s vantagepoint;
they are in reality hundreds of millions of miles apart.
Saturn is the least dense planet in the solar system. That doesn’t mean
that our solar system is populated by dumb planets, of which Saturn is just least
“intellectually challenged”. It means that Saturn’s average density – the amount
of mass it contains per unit volume - is slightly less than the density of water.
Many have pointed out that if you could find a big enough bathtub filled with
water, Saturn would float in it! Of course the downside is that it would also leave
an enormous ring….
(RIMSHOT)
Last night, another close pairing took place. The Red Planet Mars, and
mighty Jupiter, the king of planets, lay only eight-tenths of a degree apart in the
evening sky, separated by just a little more that the apparent width of a full Moon.
But far more important than Jupiter’s brush with Mars was the announcement on
June 13th that astronomers have found a twin of Jupiter!
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Well, not a twin exactly. If you consider the star around which a planet
orbits to be its parent, then our Jupiter and this newly announced planet couldn’t
be twins, because they don’t have the same parents. The newly discovered
planet orbits around another star located about 41 light years away from us in the
constellation of Cancer, the Crab. Named 55 Cancri (KAN-kree), this star is
about 5 billions years old, making it very similar in age to our own Sun.
Okay. So the new planet is very similar to Jupiter, but doesn’t have the
same parents. Maybe they could be considered cousins then? Cousins that are
almost identical. Hmmm. We’re have we heard of something like that before?
(PATTI DUKE SHOW THESE SONG CLIP)
All kidding aside, this discovery is a big deal. The first extrasolar planet –
that is, a planet around a star other than our own Sun – was announced back in
1995. Until then, we had no evidence that other stars had planetary systems.
Many were thinking that maybe our Sun was very unique in that respect. But
since then, astronomers have discovered scores of planets around other stars.
But all of these planets, up until now, have been very peculiar. All of them
have been very large – most of them many, many times the mass of Jupiter,
which is by far the most massive planet in our solar system. And they were
peculiar in other ways as well.
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In our solar system, the large gas giant planets orbit relatively far from the
Sun and have nearly circular orbit. Jupiter is the closest, orbiting at about 5.2
astronomical units. An astronomical unit is the distance from the Earth to the
Sun. So Jupiter lies about 5 times as far from the Sun as Earth does, and almost
12 years to complete one orbit. But most of the extrasolar planets discovered
have very elliptical orbits, and lie extremely close to their parent stars. So close,
in fact, that their orbital periods are often only a few weeks or even a few days. In
our solar system, Mercury orbits closest to the Sun, but it still takes 88 days to go
around once. Scientists have coined the term “hot Jupiters” to describe these
gas giants orbiting so close to their suns.
But the newfound planet around 55 Cancri does not exhibit the peculiarities
of other extrasolar planets. Lying at around 5.5 astronomical units – compared to
Jupiter’s 5.2 astronomical units – it circles 55 Cancri about every 13 years –
compared to Jupiter’s 12 years – in an orbit that is only slightly elongated.
Given 55 Cancri’s similarity to our own Sun, and this new planet’s
similarities to our own Jupiter, might this new solar system also have rocky
terrestrial planets like ours? No one can yet say. And we are not currently able
to detect Earth-sized planets around other stars. But a system to detect such
small planets, called the Terrestrial Planet Finder – TPF for short – is in the
works, and scheduled for launch sometime in the next decade. The 55 Cancri
system now seems like a pretty good first candidate for study when the TPF is
finally operational.
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For more on this week's topic, or to find out about local astronomy groups
and activities, set your browser to “ItsOverYourHead.org". Our broadcast was
written and engineered by Greg Donohue. Until next time, this is Greg Donohue
wishing you clear, asteroid-free skies and good viewing!