Transcript Slide 1

Mars Rover “Curiosity” Launch: Saturday
Landing: August 2012
Twice as long and five times as heavy as Spirit and Opportunity. Will examine
whether Mars has ever been suitable for life and to find clues about past life
forms that may have been preserved in rocks
Homework 8
Due: Monday, Nov. 28, 9:00 pm,
Exam 2: Weds., Nov. 30
Hertzsprung-Russell
Diagram
Stars do not fall
everywhere in this diagram
An HR diagram for about 15,000
stars within 100 parsecs (326
light years) of the Sun.
Most stars lie along the
“Main Sequence”
Major Factors for life on the
Surface of a Planet:
 Location, location, location:
– must lie within a star’s habitable zone
Major Factors for life on the
Surface of a Planet:
 Location, location, location:
– must lie within a star’s habitable zone
 Size is important:
– Large enough to retain an atmosphere substantial
enough for liquid water
– Large enough to retain internal heat and have plate
tectonics for climate stabilization
The Habitable Zone
An imaginary spherical shell surrounding a star
throughout which the surface temperatures of any
planets present might be conducive to the origin
and development of life as we know it.
Essentially a zone in which
conditions allow for liquid water
on the surface of a planet.
The Sun’s
Habitable
Zone
(today)
The Sun’s
Habitable
Zone
(thru time)
The Sun’s luminosity
has changed with
time.
Habitable Zones for Different Stars
Lower mass (cooler) stars
have smaller habitable zones
By contrast, the HZ of a highly luminous star would in principle be
very wide, its inner margin beginning perhaps several hundred
million km out and stretching to a distance of a billion km or more.
The size and location of the HZ depends on
the nature of the star
Hot, luminous stars – spectral types "earlier" than that of the Sun (F, A, B, and
O) – have wide HZs, the inner margins of which are located relatively far out:
To enjoy terrestrial temperatures:
Around Sirius (Spectral type A1: 26 times more luminous than the Sun),
an Earth-sized planet would have to orbit at about the distance of Jupiter
from the star.
Around Epsilon Indi (Spectral type K5: about one-tenth the Sun's
luminosity), an Earth-sized planet would have to orbit at about the
distance of Mercury from the star.
The size and location of the HZ depends on
the nature of the star
The situation becomes even more extreme in the case of a red dwarf, such as
Barnard's Star (M4: about 2,000 times less luminous than the Sun), the HZ of
which would extend only between about 750,000 and 2 million km (0.02 to
0.06 AU).
However: if planets exist too close to its parent star, the development of
life might be made problematic because the tidal friction would have led to
synchronous rotation.
 The same side of the planet will always face the star.
There are 200 billion stars in our galaxy…
…one of them is our Sun.
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The sun has eight planets…
…we know of one that has life.
WHERE TO SEARCH?
More massive, brighter stars have wider HZ.
However, massive, bright stars are much more short-lived than smaller, stars.
In the case of the massive O stars and B main sequence stars, these very
objects race through their life-cycles in only a few tens of millions of years
– too quickly to allow even primitive life-forms to emerge.
Less massive, cooler stars have narrower HZ.
But these stars live much longer than larger, more massive stars.
In the case of the low mass K and M main sequence stars, these very
objects live many tens to hundreds of billions of years – considerable time
to allow even advanced life-forms to emerge.
LIFE? Given the rate of
evolution of life on Earth, it is
possible that microorganisms
might have time to develop on
worlds around A stars.
INTELLIGENT LIFE? But in the
search for extraterrestrial
intelligence, the HZs around F
stars and later must be
considered the most likely
places to look.
Is there another Earth out there?
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Thousands of years ago, Greek philosophers
speculated.
“There are infinite
worlds both like and
unlike this world of
ours...We must believe
that in all worlds there
are living creatures and
planets and other things
we see in this world.”
Epicurius
c. 300 B.C
And so did medieval
scholars.
The year 1584
"There are countless suns and
countless earths all rotating
around their suns in exactly
the same way as the seven
planets of our system . . . The
countless worlds in the
universe are no worse and no
less inhabited than our Earth”
Giordano Bruno
in De L'infinito
Universo E Mondi
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1995
Discovery of the first planet around another star.
Didier Queloz and Michel Mayor
A Swiss team discovers a planet – 51 Pegasi –
48 light years from Earth.
Artist's concept of an extrasolar planet (Greg Bacon, STScI)
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And then the discoveries started rolling in:
“New Planet Seen Outside Solar System”
New York Times
April 19, 1996
“10 More Planets Discovered”
Washington Post
August 6, 2000
“First new solar system discovered”
USA TODAY
April 16, 1999
A useful site to keep current on discoveries:
http://planetquest.jpl.nasa.gov/
Some of the stars that have planets are
bright enough to see in the night sky…
…if you know where to look
Just how far are these new planets?
IF YOU WANTED TO RADIO HOME
from the Moon…
from Mars…
from the nearest
extrasolar planet…
it would take
one second
it would take
ten minutes
it would take
over ten years!
FOR YOUR WORDS TO REACH EARTH
But not far on a cosmic scale.
Imagine, if you shrunk our solar system to
a little larger than a quarter:
Our whole Solar
System
Our Milky Way Galaxy
would be this big
would be the size of
the United States.
And the neighborhood
where we’ve found new
planets would only be
the size of Manhattan.
How have we found
all these planets?
Using a variety of methods involving
powerful telescopes and computers
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Planets and stars orbit their
mutual center of mass.
Precise measurement of a star’s
velocity
or
change of position
tells us the extent of the star's
movement about this center of mass
induced by a planet's gravitational tug.
From this information, the planet's mass
and orbit can be deduced.
Doppler Shift
Because of the Doppler shift, light waves from a star moving toward us are shifted toward the blue
end of the spectrum. If the star is moving away, the light waves shift toward the red end of the
spectrum.
The larger the planet and the closer it is to the host star, the faster the star moves about the center
of mass, causing a larger Doppler shift in the spectrum of starlight. That's why many of the first
planets discovered are Jupiter-class (300 times as massive as Earth), with orbits very close to their
1 planet, e = 0.04
1 planet, e = 0.67
1 planet, e = 0.33
1 planet, e = 0.72
Astrometric Measurement
As with the radial velocity technique, this methods depends on the slight motion of
the star caused by the orbiting planet. In this case, however, astronomers are
searching for the tiny displacements of the stars on the sky.
Astrometric displacement of
the Sun due to Jupiter as at it
would be observed from 10
parsecs, or about 33 lightyears.
To measure the motion in the above example would require an observer in
Bloomington to be able to read the date on a quarter in Denver!
Transit Method
If a planet passes directly between a star and an observer's line of sight, it blocks
out a tiny portion of the star's light, thus reducing its apparent brightness.
Sensitive instruments can detect this periodic dip in brightness. From the period
and depth of the transits, the orbit and size of the planetary companions can be
calculated. Smaller planets will produce a smaller effect, and vice-versa. A
terrestrial planet in an Earth-like orbit, for example, would produce a minute dip in
stellar brightness that would last just a few hours.
Since Kepler’s launch:
Zero to more than 80
Earth-sized planet
candidates
Zero to 54 candidates
in the habitable zone
Area searched: 105 square degrees,
approx. 134 times the area of the Moon
At least five of the
planetary candidates
are both near Earthsize and orbit in the
habitable zone of their
parent stars."
Gravitational Microlensing
This method derives from one of the insights of Einstein's theory of general relativity: gravity bends
space. We normally think of light as traveling in a straight line, but light rays become bent when
passing through space that is warped by the presence of a massive object such as a star.
When a planet happens to pass in front of a star along our line of sight, the planet's gravity will
behave like a lens. This focuses the light rays and causes a temporary sharp increase in
brightness and change of the apparent position of the star.
Astronomers can use the gravitational microlensing effect to find objects that emit
no light or are otherwise undetectable.
Direct Imaging
Since planets do not give off their own light, observing them directly presents
formidable challenges. While the parent star is the source of light that will make
any planet visible, its glare is between a million and 10 billion times brighter than
the faint little speck we are looking for. Therefore, any direct imaging of extrasolar
planets requires methods to cover up or otherwise control the glare of the parent
star. In addition, very high resolution is needed to separate the planet from its
nearby host.
Three planets orbiting the sunlike star HR 8799. Each is
thought to have several times the
mass of Jupiter. The innermost
planet, “d”, orbits at a distance
approximately equal to
Neptune’s orbit. The system is
130 light years distant.
Stars are a billion
times brighter…
…than the planet
…hidden
in the glare.
Like this firefly.
Dusty debris disk and planet orbiting
the star Beta Pictoris. The orbit is
approximately the size of Saturn’s orbit.
The system is 50 light years distant.
Dusty debris disk and the 3 Jupiter
mass planet Fomalhaut b. Its orbit is ~
23 times the size of Jupiter’s orbit. The
system is 25 light years distant.