Transcript LifeSearch - UC Berkeley Astronomy Department Home

The Search for Extrasolar Planets
Since it appears the conditions for planet formation are common, we’d like to know
how many solar systems there are, and what they look like.
Indirect Methods:
1) Doppler shift of the star’s orbit
this is the main one so far
2) Astrometric wobble of the star’s orbit
Semi-direct Methods:
1) Transits (light blocked by the planet)
might also see phases
2) Microlensing (planet’s gravity)
Direct Methods:
1) Planet imaged directly (perhaps with coronograph)
reflected or emitted (IR or radio) light
2) Planet imaged by interferometer
Astrometry
This works best for large orbits (which
take a long time) and stars that are nearby.
Interferometry would allow very small
motions to be measured.
Precision Radial Velocity Searches
Shift is
1 part in
100
million
Discovery of Extrasolar planets
We get the orbital
period, semimajor
axis, and a lower
limit on the mass of
the planet. This can
only do giant
planets relatively
close in (but could
see Jupiter).
A Big Surprise : Close-in Jupiters
It is easiest to find a massive planet that is close to the star (it repeats
quickly and has a large velocity amplitude). The first discovery, 51
Peg, had a 4 day orbit (0.05 AU!) and the mass of Jupiter. Many are
now known, but that doesn’t mean they are most common, just
easiest to find (and present in some numbers).
Properties of the
systems found
Another surprise was that many of the
orbits are eccentric (like binary stars).
In a few cases, there are several
planets.
How did the close Jupiters get there?
1) They could have been dragged there by the accretion disk.
Corollary : some planets fall into the star!
2) They could have gotten there by interacting with another planet.
3) They could have formed there (direct collapse mechanism?)
Transits
We can watch for the dimming
of the star if the planet crosses
in front of it. This is by the
ratio of their areas: 1% for
Jupiter and 0.008% for the
Earth. This has been seen for
one case (confirming the
radial velocity detections).
HST measurement of HD209458
The Kepler Project
Transits provide the only way right now that we can reliably study
the occurrence of extrasolar terrestrial planets (none known now).
•Finds hundreds of
terrestrial planets within 2
AU of stars
•For Earth-size and larger
planets, determines:
–Frequency
–Size distribution
–Orbital distribution
–Association with stellar
characteristics
Launched 2007?
A wide-view telescope
monitors 100,000 stars in
a single field for >4 years
to detect Earth-size
planets
Sunshade
Electronics
CCD’s
Schmidt
Corrector
Thermal
Radiator
Primary
Mirror
“Microlensing” : Gravitational lenses
In principle, this method could
even see Earth-mass planets.
You have to have a huge and
long-time monitoring program
with high time resolution and
good photometric precision.
The downside is that you will only
detect the planet once, and can’t
learn anything more about it. One
tentative detection has been
claimed (but how to confirm it?).
The Problem with Direct Imaging
1) The host star is FAR
brighter (106) than any
planet (except very
young Jupiters in the
infrared).
Reflected light
Thermal emission
2) The planet is VERY
close in angle (microarcsecs) to the star, so
any stray light from the
star can overwhelm the
light from the planet.
Nulling Interferometry
You can try to keep the star
at a destructive null fringe,
while the planet will be
slightly off the fringe and
so still visible. Might be
able to reduce the star’s
brightness by a million
times?
Interferometric Missions
Perhaps a decade from now we will be able
to directly image older extrasolar giant
planets.
Darwin
Terrestrial Planet Finder
Search
Methods :
what they
can find
Eventually, imaging terrestrial planets?
Even if we can just get
a spectrum, we might
be able to detect life.
The Elements of Life
• Organic Chemistry
– By definition, involves H,C,N,O
• Most common elements (produced by most stars)
• Well dispersed and available
– Occurs even in interstellar space
• Many organic compounds found in ISM, comets, meteors
(despite extremely harsh conditions)
– Easily delivered to early Earth, or produced locally
• Biochemistry
– Requires liquid water?
– Arises naturally when basic conditions met?
• What is “life”?
– System out of chemical equilibrium which extracts energy from its
environment to maintain itself
– Energy source could be heat, light, chemical, other?
– Reliably reproduces, with opportunity for evolution
– Able to store and decode information for this
Basic Chemistry of Life (here)
From H,C,N,O (plus some trace
amounts of heavier elements like P
and Fe) are built nucleic acids,
proteins, carbohydrates, and lipids,
which can do the chemistry
needed for both metabolism and
evolution.
Photosynthesis
6CO2 + 6H2O + E C6H12O6 + 6O2
Carbon Dioxide + Water + Energy YIELDS Glucose + Oxygen
Digestion
C6H12O6 + 6O2 6CO2 + 6H2O + E
Glucose + Oxygen YIELDS Carbon Dioxide + Water + Energy
Emergence of Life on the Earth
• 0.0-0.5 Gyr
Formation and intense bombardment
– surface is uninhabitable
• 0.5-1.0 Gyr
Surface stabilizes, simple life starts
– RNA, DNA; thermophilic progenitor (chemical energy)
• 1.0-2.0 Gyr
Anerobic prokaryotes, stromatolite beds
– single-celled, no nuclei; oldest fossils formed
• 2.0-2.5 Gyr
Photosynthesis invented, free oxygen
– surface life; use of sunlight; oxygen crisis
• 2.5-3.0 Gyr
Aerobic bacteria, eukaryotes
– exploit available oxygen (more energy), cell nucleus
• 3.0-3.5 Gyr
bacteria diversify
– Keep changing the mix, experiment
• 3.5-4.0 Gyr
Sexual reproduction invented
– Evolve, baby!
• 4.0-4.5 Gyr
complex organisms appear
– Let’s get together! Let’s get it together!
The “Tree of Life”
Genetic analysis gives us a window into the distant past, and
clues on how life developed. Most of the biomass on the
Earth is still bacterial, and they are best at filling ecological
niches. Extreme life is found in amazing places.
Climate on the Earth
The Sun is getting brighter, and was 30%
fainter in the beginning. We’d be frozen now
without greenhouse gases (and really frozen
then). Somehow the greenhouse effect has been
regulated to keep liquid water on the surface. In
less than a billion years, it will be hard to stop a
runaway greenhouse on Earth (like Venus).
Habitable Zones (liquid surface water)
Many other conditions
may be “habitable”
Life here could have started at the
bottom of the ocean at volcanic vents.
Mars may have
been ready for
life first, and
seeded the
Earth. We know
rocks travel
safely between
them. We should
go and see!
Life on Earth
could be Martian
SETI : the search for extraterrestrial intelligence
• Our only real hope of detecting ET (unless they come to us) is
by listening to the radio
– Radio travels at the speed of light, over the whole Galaxy
– Radio is a low energy way to send a message
– We already have the ability
to send and receive across the Galaxy
• Where should we listen?
– Not the currently known extrasolar systems!
– Solar-type stars? Milky Way?
• How should we listen?
– Frequencies that are relatively quiet.
– How narrow-band?The “water hole”?
• What should we listen for?
– A regular carrier pattern. Complexity.
• What are the odds we will hear
something?
– The Drake equation
Orbital Chaos
70 Vir system
The Drake Equation
How Likely is Radio Contact With Extraterrestrial Intelligences?
NIC = RIC x LIC = Rstar x Pplanets x Phabitability x
Psimple life x Pcomplex life x Pradio
signals
x Lradio era
RICxLIC
rate at which civilizations appear x their lifetime
Rstar
Pplanets
Phabitability
Astronomy
rate at which stars are formed in the Galaxy
probability a star will have planets
probability a planet will be suitable for life
Psimple life
Pcomplex life
Biology
probability bacteria will arise on a suitable planet
probability bacteria will evolve into complex life
Pradio signals
Lradio era
Sociology
probability complex life will send out radio signals
total duration during which radio is sent
Evaluating the Odds Optimistically
NIC = RIC x LIC = Rstar x Pplanets x Phabitability x
Psimple life x Pcomplex life x Pradio signals x Lradio era
Optimistic Estimates
Rstar
observed rate:
Pplanets observed discoveries:
Phabitability
extreme life:
Psimple life
rapidity of life on Earth
Pcomplex life
long time on Earth
Pradio signals
who knows?
NIC = Lradio era/100
10 per year
0.5
0.5
1.0
0.2
0.02
pick your favorite duration…
So if Lre is greater than a few hundred years, there’s
probably somebody out there.
Lre needs to be a million years
for them to be neighbors
(meaning within 1000 ly).
The Galaxy’s a big place, and
its been around a long time!
Allen Array
…and so, we are listening!
You can help too! Download
seti@home
(right here in Berkeley).
2005?
Rapid Prototype Array
Arecibo
(Puerto Rico)
Evaluating the Odds Pessimistically
NIC = RIC x LIC = Rstar x Pplanets x Phabitability x
Psimple life x Pcomplex life x Pradio
signals
x Lradio
era
Pessimistic Estimate
Rstar
observed rate:
10 per year
Pplanets observed discoveries:
0.1 (no terrestrials known)
Phabitability
extreme life:
0.01 (surface liquid water)
Psimple life
rapidity of Earth life 0.1 (we got lucky)
Pcomplex life
long time on Earth
0.01 (looks tough)
Pradio signals
who knows?
0.001 (what good are radios?)
NIC = Lradio era/100 million
duration doesn’t much matter…
Pessimistic Conclusion:
There’s nobody home (except for us!).
Let’s be careful, live long, and prosper!