When will we find the first “earthlike planet” in the
galaxy?
According to some mainstream news reports we have found them
already -- again and again and again.
The problem is one of semantics. If an exoplanet is close
to the same mass of Earth it’s called "earthlike" in press releases and the news
media.
The one that got lots of press a couple weeks ago was a world
called CoRoT-7b, located 500 light-years away. The planet tips the scales at
4.8 Earth masses and is a little less that twice Earth’s diameter and the least massive exoplanet to date that has ben found orbiting a normal star.
Ebullient NASA scientists today released a stunning portfolio
of full color imagesof the
universe taken with the newly upgraded and refurbished Hubble Space
Telescope.
The new Hubble camera, called Wide Field Camera 3, is“panchromatic.” This is an old term
from the days of photographic filmthatdescribes a black-and-white emulsion
with a sensitivity to all colors of
light. The WFC3 goes even further with sensitivity stretching from
ultraviolet to near-infrared wavelengths. Call it Super-Technicolor!
Like B&W film used in the Technicolor motion picture process, a Hubble exposure starts out as monochrome. Next, separate pictures are taken though different color filters and assembled into a full color image.
My favorite early-release WFC3 photo is shown here. It is a view looking deep into the heartof the globular star cluster Omega
Centauri.
The bright red star Betelgeuse in the winter constellation Orion the
Hunter is sure to get a giggle in introductory astronomy college classes. Older
students may remember the 1988 Tim Burton film Beetle Juice with comic Michael
Keaton. And, it’s fun to tell younger students that Betelgeuse is Arabic for “armpit
of the giant” (which is actually a mistranslation).
The latest data from the Very Large Telescope in Chile telescope
show that this star is nothing to laugh at. It is one bad-ass supergiant, with a dragon’s breath plume of gas, and boiling monster bubbles of gas – yuck!
Placed
inside our solar system it would swallow Earth and the other inner planets and
extend all the way out to Jupiter’s orbit. The volume of space such a monster
star engulfs is simply inconceivable. Imagine the sci-fi film: The Star That Ate My Planet!
For years I’ve hypothesized that binary planetary systems
exist, and that these are the places to go looking for space-faring civilizations.By binary planetary system I mean a pair of gravitationally
bound stars, each with its own independent family of worlds.
There could be habitable planets around each star in the
binary system, if the stars are far enough apart. If technological
civilizations independently evolve around both stars, they could actually travel across space and
visit each other. This would be a powerful motivation to build the fusion-powered
rockets needed to travel back and forth within a reasonable amount of time.
Astronomers have now found such as system, but we’ll have to
wait another 4 billion years before an alien civilization manifests itself. Why?
Because the double star is just a few millions of years old, and if Earth
is any example, it takes a long time to evolve intelligent beings.
Planet
sleuths Michel Mayor and his team became science news media stars with today’s announcement
of the detection of the smallest exoplanet to date. Tilting the scales at only 1.9
Earth masses is Gliese 581 e, the fourth member of a planetary system located only 20.5 light-years away in the spring constellation Libra.
To sci-fi fans this is a well-know quote from Arthur C.
Clarke’s novel2001: A Space Odysseywhen lone astronaut David Bowman falls into an alien
stargate located on the strange Saturnian moon Iapetus.
But now reality has zoomed past fiction with a picture that
would challenge even Clarke’s fervent imagination.
Last week the protective light cover was jettisoned off NASA’s Kepler
observatory, taking it another step closer to conducting a historic census of Earth-like planets that lie in the direction of the Summer Triangle.Kepler is now eight times farther from
Earth than the moon.
Kepler’s survey will be completed in less than four years. Then astronomers
will proudly announce a statistical value for the abundance of Earth-like
planets in our Milky Way galaxy.
But, coincidentally there is another, largely unpublicized study going on over the same time
period that potentially would eclipse the Kepler finding and reshape the
astronomical research agenda, with the headline:
Earth-like Planet Found Around Nearest Star to Our Sun
The closest star system to our sun, Alpha Centauri is only 4.3 light
years away. It may be a triple system, with two sun-like stars orbiting each
other at a distance ranging from one to three billion miles. (A small red dwarf,
Proxima Centauri, is on the periphery of the system and may or may not be
gravitationally bound to it)
The two stars have high concentrations of heavy elements, which is
characteristic of stars that are born surrounded by dusty, planet-forming disks.
And, the system is as old as the Sun.
Last May a team of astronomers from U.C. Berkeley, San Francisco State
University, and the SETI Institute started a five-year observing program of
Alpha Centauri B, the fainter of the two stars.The team, led by Deborah Fisher, is intensively monitoring the star using the
1.5-meter telescope at the Cerro Tololo Inter-American Observatory in Chile.The team is trying to measure if there
is a telltale signature of an Earth-mass planet that is inducing a wobble in
the star that would be measured by the Doppler effect. The star’s offset from
its nominal center-of-gravity would be only a few inches!
The optimism of finding a terrestrial planet is based on a computer
model where a group, led by Greg Laughlin, ran repeated simulations of how planets would form around
Alpha Centauri B over a 200 million year period. With each change in the initial conditions of the protoplanetary gas
and dust disk, each simulation led to the formation of a different planetary
system. In every case, however, a system of multiple planets evolved with at
least one planet about the size of Earth. In many cases, the simulated planets
had orbits lying within the habitable zone of the star.
However, another simulation of Alpha Centauri planet formation by
Philippe Thebault or Stockholm Observatory concludes that the dynamics of the
Centauri binary system are hostile to planetesimal accretion required to build
planets. In the complex gravitational field between the two stars, planetesimals
would be accelerated to the point where they would smash together rather than
gently merge. The only way out of this dilemma would be to form a rocky planet closer
in to one of the binary stars and have it migrate out to the Earth-sun separation
distance, he reports.
To me the bottom line is that we don’t know until we look. Predicting
the outcome of an evolving circumstellar disk in a binary system is filled with
so many variables, it’s like predicting the shape of the swirl cream will make
when poured into hot coffee.
Planet Game Changer
The discovery of an extrasolar terrestrial planet so nearby would send
seismic waves across the astronomy community. And it would energize the public about
space exploration. The kicker is that that any such planet, if inhabited, has had
4 billion years available to undergo Darwinian evolution.
This would accelerate popular interest and Congressional support for
building a large enough optical-IR space observatory that could spectroscopically
sample the planet’s atmosphere. At 4.3 light years away, an 8-meter mirror
should do it. Attempts would also be made with the 6.5 meter James Webb Space
Telescope.
If observations confirmed the planet’s atmosphere has biotracers, such
as oxygen, ozone, nitrous oxide, methane, and chlorine, there would be a push
to build a dedicated interferometric array of space telescopes to make an image of the planet.
The picture would be no more than least one or two-dozen pixels
across. That would be enough to monitor the planet. Observations could reveal
the waxing and waning of continents and oceans as the planet rotates, and the
changing tapestry of weather patterns.
A number of SETI
experiments would monitor the planet for evidence of alien telecommunications.
By the dawn of the next century, there would be one or more attempts
by various countries (unless the cost is so exorbitant we all pool resources) to
send a very low-mass probe to fly by the planet at about 1/10th
the speed of light. It would collect tons of close-up data in just a few
precious hours it navigated the system before hurtling back into interstellar
space.
Such a mission would have a cruise phase of only 40 years. So, unlike
other interstellar missions, the builders of the craft could live long enough
to see data returned. Propulsion engineering for such a probe, perhaps boosted
by an interstellar laser or a fusion ramjet engine, should be mature within the
next century.
Depending on the results, the next big push would be to have a probe
land on the planet and study the biology in-situ. By the time we are ready for
such a mission we will have matured the required artificial intelligence and
nanomachine technology. The autonomous probe would, upon arrival, build a variety of nanobots to carry our planetary exploration and direct the survey.
Such a discovery would be terribly ironic and humbling. The question of
life is space has had a high“giggle
factor” through most of astronomy’s history, and has been cast into doubt by such concepts as the Rare Earth hypothesis. But to have an inhabited world so
close by would profoundly alter on our view of universe.
Once you know what you’re looking at, then you know what
you’re looking at.
No, I didn’t borrow the Twitter-sized bit of wisdom from
baseball legend Yogi Berra.But it
does summarize two exoplanet “retro-discoveries,” one even announced today.
Now that we’re getting the lay of the land when it comes to
identifying planets orbiting other stars, astronomers are looking back into
archival telescopic data to see if any exoplanet reared its head in decades-old
observations, but was simply overlooked.
Astronomers sifting through the vault of Hubble Space Telescope pictures believe they’ve nabbed a planet buried in observations taken in the late
1990s. They knew just where to look for the giant planet, because it was
positively identified in 2008 in ground-based observations. But other transiting planets could be
hidden away in a near-infrared Hubble dataset of 200 selected stars.
Another archival piece of exoplanet evidence goes back to November 10, 1981, when astronomers at the Geneva Observatory
in Switzerland noticed a small momentary drop in the brightness of the star Beta
Pictoris. But, unlike the case of other known exoplanets that whirl around their stars
in a matter of days, the mysterious “blink” didn’t repeat in the many years of observations
that followed.
In 1998 the event was proposed to be the passage of a large
comet or planet in front of the star (transit), but no additional transits
had been detected even by then. This suggested that if there were a planet it might
be on a long period orbit, like Jupiter (12 years) or Saturn (29.5 years).
Fast-forward to 2008 when French astronomers announced that
they may have an image of the suspected planet orbiting Beta Pic. They
accomplished this by reanalyzing infrared observations of the star first made
in 2003 with the Very Large Telescope in Chile. It is closer to its star than
any photographed exoplanet previously reported.
The candidate planet lies at about the orbital distance of
Saturn from the sun, and has been calculate to be in a roughly 18-year orbit, based on
its distance from the star. If those numbers are correct, researchers say it
could be the same object that was seen passing in front of the star in 1981.
The 12-million-year-old star has been a poster child for
exoplanet searches since a dusty disk of debris – presumably formed from
planet collisions- was first photographed around it in 1983. The disk is tilted nearly edge-on to Earth. Likewise so should the planet's orbit be similarly inclined, allowing for transit observations. But it has taken
nearly three decades to nail down the presence of a long-period Jupiter-class planet.
Using a new image processing technique that is very effective
at suppressing the glare of the parent star, a team of Canadian astronomers announced
today that they found the telltale glow of a planet first discovered in 2007,
in Hubble archival data taken in 1998. The giant planet is young and hot, but
still only 1/100,000th the brightness of its parent star (by comparison, cooler
Jupiter is one-billionth the brightness of the sun).
Hubble’s Near Infrared Camera and Multi-Object Spectrometer
(NICMOS) has looked at over 200 other stars, where is uses a coronagraphic to
block out the light of the star to search for the feeble glow of planets. The
team plans to look for undiscovered planets in the NICMOS archive dataset, and
do follow-up observations with ground-based telescopes on any candidates that
pop up.
So, there’s planet gold in “them thar” databases. Potentially
undetected exoplanets could be encouraging evidence that unseen worlds are even more abundant, and are scattered across the galaxy.
The newly launched Kepler observatory’s search for Earthlike worlds
will provide simply a census of how common Earth-sized planets are in the
galaxy. In about three to four years Kepler scientist will announce the
fraction of sunlike stars with planets in their survey. In theory it could be
anything from 0 to 100 percent. The final value might be somewhere in the
middle -- say 50 percent if we split the difference – but right now it’s
anybody’s guess.
If the Kepler percentage is high, then astronomers will be
encouraged to do a search for nearby Earthlike planets within a distance of a few tens of
light-years. This will be tough to do without another space platform optimized
for poking around our stellar backyard. Kepler’s Earthlike planets will be
hundreds of light-years away – too far for much further scientific scrutiny.
Short of building a new dedicated planet-hunting observatory in the next decade, an
alternative possibility is to use NASA’s James Webb Space Telescope, which is
already under construction for a 2013 launch. Webb will have greater
sensitivity than Hubble and have equivalent eagle-eye sharpness at infrared
wavelengths.
For the added cost of a free-flying spacecraft to block out starlight,
Webb might be capable of identifying nearby Earths. The occulter would be a
parasol-like satellite flying roughly 40,000 miles in front of Webb to blot out
the glow of the parent star, allowing the feeble reflected light of a planet to
be collected.
We’ll need to wait until the 2020s – at the earliest – to have a big
enough telescope in space capable of actually observing an exo-Earth in enough
detail to measure the chemical composition of its atmosphere to look for
biomarkers of life processes, such as oxygen and methane.
The telescope would minimally need to have a mirror 8-meters across
to be able to collect enough light from an exo-Earth – which is one ten billionth the brightness of its parent star. And even that might not be adequate if
the Kepler census shows that Earths are rare, and hence, proportionally farther
away.
A mirror 16-meters across would offer a much greater chance of success.
It would provide a robust sample of many Earthlike planets out to nearly 100
light-years. This volume of space could contain as many as 1000 Earths,
depending on Kepler’s census.
On the drawing boards is a multi-segmented space telescope called
ATLAST (Advanced Technology Large Aperture Space Telescope). It would be the
true son-of-Hubble, with an ability to see celestial objects with as much as
ten times greater sharpness and as much as 100 times greater sensitivity than
even Hubble. So it would be to Hubble what Hubble is to ground based
telescopes! ATLAST would join its kid brother, Webb, at a gravitationally
stable space “parking lot” beyond the moon, Lagrangian point 2.
Using an onboard coronagraph, or free-flying occulter, ATLAST would
collect light from exo-Earths and disperse it into a color spectrum. The spectral
fingerprint of an inhabited planet would yield absorption lines in water vapor,
oxygen, carbon dioxide, ozone, and methane.
ATLAST would also make precise measurements of fluctuations in a
planet’s brightness and color as the planet rotates. An Earthlike planet would
yield the photometric signature of clouds, oceans and continents. The more
variegated the planet, the more likely it has ecological niches for a diversity
of life forms.
Regardless of the number of exo-Earths, it’s a safe bet there will
be tremendous variability among them. First, they will be scattered along
different ages. Earth’s atmosphere was markedly different 2, 3, and 4 billion
years ago. Secondly, the exo-Earths will also vary in mass, the majority fitting the
definition of “super-Earth” by being several times Earth’s mass. This alone
will likely mean there is a diversity of types of atmospheres in
disequilibrium.
Therefore, the first exo-Earth probed probably will not have an
atmospheric composition that is exactly a clone of Earth’s. We simply couldn’t
be that lucky, given the likely staggering diversity of planet chemistry driven
in part by a vast range of surface geologic activity. Scientists will debate whether the exoplanet’s unusual atmosphere is a product of bizarre
geologic processes or the action of life on the surface.
So, to properly follow up on the Kepler census, my vote is for a
16-meter space telescope to provide us with a large enough sample of planet
atmospheres. Based on the surprises we’ve had any time we boost our observing
capabilities, I predict that exo-Earths will present a befuddling array of
environments. With a bigger light collecting area, ATLAST could look at more
planets and would spend less observing time on each planet because of the higher
light-collecting efficiency. What’s more, building something twice the
diameter of an 8-meter mirror does not mean it’s twice the cost.
So the challenge for astronomers will be like buying a car where the
salesman says the if you buy the cheaper car, he can’t guarantee it will get you
all the way to the next town. A telescope the magnitude of ATLAST comes along
once in a generation, so when the decision is made, let’s make sure it will at
last allow us to answer the ages-old question of whether there is life among the stars.
A titanic battle of the giants is looming in intergalactic
space. The neighboring spiral galaxy in the constellation Andromeda is falling
toward us at a speed of one million miles per hour. At that velocity it will
plow into our Milky Way only a few billion years from now.
This close-to-home galaxy mergers ranks among the biggest
big bang-ups in the universe, and it's dramatically illustrated in
the Discovery Channel's upcoming TV series -- Cosmic Collisions.
Wait -- galaxies collide? At first glance this may sound counterintuitive; after all, Andromeda is more than a million times farther away than the nearest star to
our sun, so it seems there would be no chance of them ever colliding.
But consider that our Milky Way galaxy is 100,000 light
years across, and is separated from Andromeda by 22 galaxy diameters (2.2
million light years). By comparison, the sun is nearly a million miles across.
The nearest star, Proxima Centauri, is 100 million solar diameters away (24
trillion miles).
Put another way, if the sun were the size of a hockey puck,
the nearest star would be -- to scale -- 4,700 miles away! But if the Milky Way were the size of a hockey puck,
the Andromeda galaxy would be another hockey puck located a little less than
six feet away. That's within gravity's striking distance for a pair of galaxies
that each weigh a few trillion solar masses.
Supercomputer simulations that model the galaxies'
gravitational pull on each other give us a preview of the close encounter
that's coming (the actual collision will span one billion years). It looks sort
of like what happens in the children's poem the Gingham Dog and the Calico Cat, who
rip each other apart. The stars are scatted like a fallen box of marbles and
the galaxies lose their trademark spiral shape and finally morph into a giant
elliptical galaxy.
This is more than theory though. Over the past few decades
astronomers have collected dramatic snapshots of pairs of galaxies colliding,
or more politely put, interacting.
Sometimes more than two galaxies get into the act. One of
the most spectacular examples is the mosh pit called Stephan's quintet of five
galaxies mixing it up. Hubble Space Telescopes has shown that this was more the
case in the early universe because it was smaller, and galaxies bumped intro
each other more often.
When you view computer simulations, it looks like the end of
the line for anyone living in a colliding pair of galaxies. But this couldn't
be further from the truth.
Remember how comparatively far apart stars are? Even if another galaxy passed through the
Milky Way the sun would never collide with another star.
But what other bad things might happen to our solar system
in a galaxy collision? On closer
inspection it's sort of like the aftermath of a Roadrunner vs. Coyote cartoon. Despite all hell breaking
loose in the dynamics of the collision, the vast majority of earthlike planets
would go on with life as usual.
The biggest consequence of a galaxy collision is that stars
are tossed along gravitational tidal arms that stretch out each galaxy to
resemble an s-shaped fan blade. During the bang-up our solar system may find
itself rapidly relocated in one of these long arms. Our distant descendants
would have a truly bird's-eye view of the galaxy makeover in progress.
Billions of new stars would be born is a firestorm of
star birth triggered by the impact's compression of cold hydrogen in each
galaxy. Five billion years from now the sky will be ablaze with gem-like
clusters of brilliant hot blue stars and glowing nebulae. It will be a great
time to be an astronomer.
The downside of a having a beautiful faux Van Gogh Starry
Night sky is that there will be many more massive stars that explode as
supernovae and belch out lethal radiation. Mergers of double stars may cause
gamma ray bursts that would barbecue planets caught in their death-ray beam
within a few hundred light-years. But given the vastness of the galaxy this is not
as doomsday-ish as it sounds. The large majority of stars and planets will go
on unscathed.
Another scary consequence is that the 3 billion-solar-mass
black hole in the core of the Milky Way would merge with the 4 billion-solar-mass black hole in Andromeda. This would send gravitational waves rippling
across the galaxy that would monetarily pinch Earth's diameter by one-inch,
like squeezing a soccer ball.
Gas falling toward the monster black hole would be heated
and expelled along a blowtorch-like beam of high-speed particles and radiation
called an extragalactic jet. Any unlucky planet caught in the beam could see
much of its atmosphere stripped away.
However, a huge teardrop shaped solar wind “bubble” around
our sun could serve as a buffer to deflect the jet particles, depending on the
sun's distance and beam intensity. Also, a laser-narrow jet would only affect a
small fraction of the stars in the Milky Way.
Probably the biggest risk to Earth would come from having a
near-passing star gravitationally perturb the Oort cloud of comets. Like
shaking apples out of a tree, the dislodged comets would fall into the inner
solar system. A shower of wayward comets would bombard Earth, causing global
mass extinctions.
Now, to avoid any more melodrama, all of this could very well be a moot
point -- the sun is scheduled to burn out in 5 billion years, just when the
intergalactic fun is beginning.
The other unknown is that we won't know if the galaxy close-encounter
will be a head-on collision or glancing blow. Astronomers do not precisely know
if the Andromeda galaxy is on a trajectory causing it to swing wide of us. In
fact the Milky Way and Andromeda may simply orbit each other for quite some
time, like two Sumo wrestlers sizing each other up.
The bottom line is that the universe is indifferent to our
fate. Trillions of new planets could be born out of such a galactic close
encounter, even if it did mean demolishing old homesteaders like Earth.
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