快猫短视频

LAST YEAR looked set to be another bad year for astronomers scouring the
Galaxy for alien planets. In April, Gordon Walker from the University of British
Columbia in Vancouver published a paper detailing a fruitless 12-year hunt for
solar systems around 21 nearby stars. Other groups trying their luck with
different stars had nothing to report. Perhaps, some wondered gloomily, our
Solar System could be unique.

But in October everything suddenly changed. Michel Mayor from the Geneva
Observatory announced that he and student Didier Queloz had discovered a planet
orbiting a star just like the Sun. The sensational news hit the headlines around
the world, and almost immediately pointed the way to the discovery of yet more
planets. 鈥淚t鈥檚 as if everyone was waiting for our discovery,鈥� says Queloz. 鈥淚t鈥檚
like a bottle of champagne. One team discovers a planet and the bottle just
explodes鈥攐ne planet, two planets, three. We just gave it the first
办颈肠办.鈥�

Ironically, Mayor and Queloz did not rate their chances of finding a planet
very high when they started their search in September 1994. Their goal was to
look for failed stars called brown dwarfs circling companion stars. According to
theory, brown dwarfs form from a collapsing cloud of gas and dust, just as stars
do, but are too small to shine.

Mayor and Queloz鈥檚 search technique, pioneered by Walker, involved looking
for the tiny wobbles in the positions of stars that would be caused by the
gravitational tug of orbiting objects. The slight movement would show up as
Doppler shifts鈥攖iny periodic changes in the frequency of light from the
star as it moved towards and away from the Earth. The size of the frequency
change gives the amplitude of the star鈥檚 movement, which in turn gives a handle
on the mass of the orbiting object. Its orbital period is exactly equal to the
period of the frequency variations.

Mayor and Queloz began by monitoring 142 Sun-like stars every two months. By
February 1995, Queloz realised that the star 51 Pegasi in the constellation
Pegasus was showing just the kind of frequency shifts they would expect if
something was in orbit around it. But, intriguingly, the shifts suggested that
the object would have to be relatively small鈥攁round half the mass of
Jupiter. Something so small had to be a planet rather than a brown dwarf. Most
theorists believe the lower mass limit for a brown dwarf is around 10 to 20
times the mass of Jupiter. Less than this and the gravitational pull is not
enough to counteract the outward pressure, which increases as the gas collapses
and heats up.

At first, Mayor suspected that the planet might be an illusion caused by
instrument errors. But the same thing happened in the next observing run in
March 1995. The team then had a frustrating four-month wait as 51 Peg
disappeared into the glare of the Sun. In July, both researchers returned to the
telescope, armed with a prediction of the exact light signals they should see if
a planet was really there. Their predictions were spot on.

In a spin

Mayor and Queloz treated themselves to champagne. But before they could
release the news, they still had to establish whether the signal really came
from a planet. After all, a host of other things could be responsible. For
instance, the star itself could be pulsating, or it could have giant star spots
that appeared and disappeared as it rotated, making it seem as if the whole star
were moving backwards and forwards. The team ruled out the first option by
establishing that there were no other tell-tale signs of pulsation such as
changes in the colour or intensity of the starlight. They also discovered that
the star was spinning far too slowly for star spots to account for a periodic
change every four days.

They had more work to do to confirm the mass of the object. The Doppler
technique gives only a lower limit for the mass of the orbiting object; if the
orbit is not exactly edge-on, as seen from the Earth, the mass could be higher,
perhaps high enough for it to be a brown dwarf. To rule this out, the
researchers used the shape of the spectral lines to work out the geometry of the
system. It turns out that the lines should be broad if the star is rotating
edge-on as seen from the Earth, and narrow if the star is tilted鈥攁nother
example of the Doppler effect in action. Mayor and Queloz discovered that the
lines from 51Peg were broad and concluded that the star was not particularly
tilted. Because objects tend to orbit around the equator of the star rather than
over its poles, the object鈥檚 orbit could not be significantly tilted as seen
from the Earth. In other words, the mass of object was indeed not much bigger
than half the mass of Jupiter.

By August, Mayor and Queloz were confident enough to submit their paper to
Nature, though they were still nervous about what the referees would
think. The problem was that although the new planet was comparable to Jupiter in
size, it was astonishingly close to its parent star. It lay at a distance of
only 0.05 astronomical units, compared to 5 astronomical units for cold, distant
Jupiter (an astronomical unit is the distance from the Sun to the Earth). Its
orbit took just four Earth days compared to Jupiter鈥檚 stately 12 years.

Mayor鈥檚 first worry was whether a Jupiter-like planet could survive so close
to its star. Jupiter is a giant ball of gas that probably has a small rocky
core. Astronomers believe that any planet whose size comes even close to
Jupiter鈥檚 must be a gas giant too. But would the heat from the nearby star draw
off the gas, and destabilise the whole planet?

To answer that question, Mayor asked several theoreticians how close Jupiter
could be to the Sun before it became unstable. None of them knew. But the answer
finally came from Adam Burrows of the University of Arizona. On hearing of the
problem from an ex-student of Mayor鈥檚, he ran simulations of planets at all
distances from a Sun-like star. Within 24 hours he came back with the news that
anything farther away than 0.04 astronomical units would be stable. At 0.05
astronomical units from its star, Mayor鈥檚 planet could comfortably exist.

Mayor decided that the time was right to present the discovery to his
colleagues. So at an October meeting in Florence, he announced the results to a
packed hall. The press were ecstatic and the news spread like wildfire. 鈥淚 had
hundreds of phone calls, faxes, e-mails,鈥� says Mayor. One e-mail was from a
6-year-old American boy wanting to know if Mayor had visited his planet yet. An
irreverent French newspaper columnist reported the concerns of Piero Coda, a
Roman priest, about whether any inhabitants of the new planet would need to be
saved from the taint of original sin.

The same excitement spread among Mayor鈥檚 fellow astronomers鈥攊ncluding
long-time planet searcher Geoff Marcy from San Francisco State University. Along
with his colleague Paul Butler from the University of California at Berkeley,
Marcy had been monitoring 60 stars for seven years in the hope of finding a
planet, and had just begun to look at another 60 with higher resolution. As soon
as Mayor announced his discovery, Marcy and Butler raced out to their telescope
to check it out. The object was so strange that they were convinced it would be
a false alarm. But to their amazement, they saw the same signal.

But why had they seen no planets in their own search? Like most astronomers,
they had fallen into the trap of assuming that any alien solar system would look
pretty much like our own. If a planet was big enough to detect, they thought, it
should lie far from its parent star and take many years to complete a single
orbit鈥攋ust like Jupiter and Saturn. They had decided it would take many
more years of stargazing before they saw a planet, and so had not even begun to
analyse their data.

Having confirmed Mayor and Queloz鈥檚 sighting, however, Marcy and Butler
turned to their data and began number-crunching in earnest. To their
frustration, they discovered planets that had been sitting around on their
computer hard discs waiting to be noticed. Now they raced through their
analysis. Robbed of the chance to announce the first ever planet detected
orbiting a Sun-like star, they were determined not to be beaten to any others.
In January, they reported two new planets鈥攐ne orbiting 70 Virginis and the
other orbiting 47 Ursae Majoris (快猫短视频, Science, 27 January, p
17). These lie at about 0.5 and 2.1 astronomical units from their stars
respectively. Then in April the team announced another planet鈥攐rbiting Rho
Cancri, a star in the constellation of Cancer, and in the last couple of weeks
yet another orbiting the star Tau Bootis.

Another planet was found in January lurking deep inside the dusty disc
surrounding Beta Pictoris (see 鈥淒usty discovery鈥�). And as 快猫短视频
went to press, George Gatewood of the University of Pittsburgh was planning to
announce the unconfirmed discovery of at least one other planet at the June
meeting of the American Astronomical Society in Madison, Wisconsin. Gates and
his colleagues studied data for the star Lalande 21185, the fourth nearest to
the Sun. They say that the red dwarf star appears to have a planet with an orbit
period of about 5.8 years and a mass of 0.9 times that of Jupiter. The tally for
new planets now stands at seven, and there are rumours of more in the
pipeline.

Already, the newly discovered planets are shaking long-established theories
of how solar systems form. Existing theories give a neat picture of how our
Solar System formed and explain why the small rocky planets鈥擬ercury,
Venus, Earth and Mars鈥攊nhabit the inner regions while the cold gas
giants鈥擩upiter, Saturn, Uranus and Neptune鈥攃ircle much further out.
Pluto, the outermost planet, is a rocky exception, believed to come from an
outer ring of asteroids called the Kuiper belt (鈥淎ll aboard the Pluto Express鈥�,
25 May, p 34).

Tradition has it that the Sun formed from a collapsing interstellar cloud of
gas, ice and dust. The material around the young protostar flattened into a
spinning disc. In the outer parts of the disc鈥攂eyond the solar 鈥渟now line鈥�
where temperatures became low enough for ice to remain solid鈥攄ust and ice
particles collided and agglomerated into planetary cores several times the
present size of the Earth. So heavy were these cores that they dragged in gas
from the disc, wrapping themselves in a deep gassy mantle.

Closer to the Sun there was no solid ice to build heavy cores. Instead dust
particles slowly agglomerated to form the small rocky planets. Meanwhile, the
gas and dust of the disc was gradually disappearing鈥攆alling onto the star
or colliding with other particles and spinning off into interstellar space,
perhaps carried along with material blowing out from the newborn star. So before
the rocky planets were heavy enough to pull in gas and grow to the size of their
giant siblings, the disc had disappeared.

But how then could all the newly discovered giant planets have formed? They
all lie between about 0.05 and 2.5 astronomical units from their stars, well
within the snow line of each. In January, theorist Alan Boss from the Carnegie
Institution of Washington reported that he had stretched his models to the
limit, but could form no Jupiters closer than 3 astronomical units to the
star.

Last exit

Several theorists began to explore an alternative explanation. Perhaps the
new planets did indeed form as gas giants out beyond the snow line, but then
moved inwards. Models show that if the disc hangs around for long enough,
gravitational interactions between the disc and the planets could drag them in
towards the central star. If this is right, it would simply be a matter of
chance that it did not happen in our own Solar System. 鈥淚t鈥檚 a question of the
disc staying around long enough in our Solar System to form Jupiter and then
exiting stage left,鈥� says Boss. 鈥淧erhaps the disc around 51 Peg didn鈥檛 want to
get off stage.鈥�

But why should the inward motion eventually stop, leaving the gas giants
close to their stars? In April, theorist Doug Lin from the University of
California at Santa Cruz outlined some alternatives in Nature, but the
details have not been thrashed out. 鈥淚 don鈥檛 think any of these ideas makes
complete sense yet,鈥� says Burrows.

If the new planets did move inwards during their formation, it leaves little
hope for finding smaller, Earth-like planets in their vicinity. The process
would probably have played havoc with small rocky planets, flinging them out of
the solar system or plunging them into the star.

There is already an intriguing hint that this has happened, according to
Marcy. He points out that the three stars with the closest planets鈥�51 Peg,
Rho Cancri and Tau Bootis, all contain significantly more heavy elements than
the Sun. Heavy elements are concentrated in the dusty particles that form
planets, so this could simply mean that a dustier initial cloud is more likely
to lead to planets. But it could also be the last remaining sign of planets that
have been swallowed by their stars. A handful of rocky planets wouldn鈥檛 make
much difference to the overall composition of a star鈥攖he Earth, for
instance, is only around a millionth the mass of the Sun. But, says Marcy, it is
possible that the heavy elements could remain in the star鈥檚 outer zone and
create a detectable difference.

Another possible explanation for the positions of the new planets is that
close encounters between two or more planets could have hurled them in opposite
directions, some ending up close to the star and others at very distant orbits.
Various groups are now working on simulations to test this idea.

In the meantime, there has been plenty of speculation about whether any of
the planets might play host to life. The 70 Vir object was originally the hot
favourite because at 0.5 astronomical units from its star, it would probably
have a temperature of about 80 掳C. So it is the only one that would be able to
support liquid water, an essential ingredient for life. Though gas giants have
no surfaces as such, Marcy suggested that complex molecules could be floating in
the planet鈥檚 vast atmosphere. Many newspapers took the idea further.
Time magazine even published an artist鈥檚 impression of the planet鈥檚
putative aliens.

But most astronomers were highly sceptical about the possibilities of life on
a planet such as this. 鈥淭here鈥檚 nothing wrong with dreaming, but let鈥檚 try to
keep cool,鈥� said one dryly. And now astronomers are wrangling about whether the
object that orbits 70 Vir even deserves to be called a planet. Unlike all the
others, the 70 Vir object has an eccentric orbit. If it had formed in a disc,
the orbit should have been circular as collisions between disc particles quickly
iron out any eccentricity. Also, the 70 Vir object is on the heavy side for a
planet, with a mass of at least 6.5 Jupiter masses, and if its orbit is tilted,
it could be heavier.

Because of this, most astronomers suspect that the companion to 70 Vir is
really a brown dwarf that formed through the gravitational collapse of a cloud.
This would dampen hopes of finding Earth-like planets鈥攁nd ultimately alien
life鈥攊n that star system. If the 70 Vir object turns out to be a brown
dwarf, there would be no evidence that the system ever possessed a disc of the
kind that could create planets like our own.

However, Marcy is adamant that the identity of the 70 Vir object should
remain an open question. He points out that we still know little about brown
dwarfs鈥攁stronomers have found few convincing candidates so far, and most
seem to be much more massive than the minimum size that theories suggest. 鈥淚
think it鈥檚 premature to assign any terminology to these guys,鈥� he says. 鈥淐all
them what you like鈥攃all them brown dwarfs, call them toasters. Nature is
clearly manufacturing some low-mass guys that are defying our expectations.鈥�

Marcy is also wary of assuming that formation in a disc rules out eccentric
orbits. 鈥淚t could well be that discs are cleverer than we are,鈥� he says. Burrows
agrees. 鈥淭here could be a number of things that we just haven鈥檛 thought of,鈥� he
says. 鈥淲e need to keep an open mind because we鈥檙e starting to see things that we
really hadn鈥檛 predicted.鈥�

Perhaps this is the best lesson to take. As more planets show up, creative
thinking will be in order. And the next few years will certainly see more
discoveries. Marcy and Butler still have one object in their data that deserves
a closer look. After that, they plan to trawl the entire dataset more carefully.
They also plan some observing with the 10-metre Keck Telescope in Hawaii.

Meanwhile, Mayor and Queloz have their eyes on four or five promising
candidates. And many more groups are joining the hunt. At the January meeting of
the American Astronomical Society in San Antonio, Texas, there was extravagant
talk. 鈥淲e are at the gateway of a new era,鈥� declared Marcy. 鈥�1995 will be looked
back on as a wonder year,鈥� proclaimed Neville Woolf of the University of
Arizona. They could both be right.

* * *

Dusty discovery

IN JANUARY this year, Chris Burrows from the Space Telescope Science Center
in Baltimore reported the discovery of a planet orbiting Beta Pictoris in the
southern constellation of Pictor. Unlike the other new planets, this is
something of a one-off, since its presence is revealed by a slight warp in the
dusty gas disc surrounding the star.

Burrows says that the planet tugs on kilometre-sized bodies buried in the
disc, causing them to wobble above and below the plane, with the dust and gas
following suit. The warp occurs, he says, because the outer part of the disc has
not yet had time to react to the presence of the planet.

From the size and position of the warp, Burrows can estimate the
gravitational pull of the planet. He says it must lie somewhere between the
inner edge of the disc and the star, and have a mass of a twentieth to 20 times
the mass of Jupiter.