Admin. 11/28/171. Class website http://www.astro.ufl.edu/~jt/teaching/ast1002/2. Optional Discussion sections: Tue. ~11.30am in Bryant 3; Thur.

~12.35pm, start in Pugh 170, then Bryant 33. Office hr: Tuesday 12.30-1pm; Wed. 12.30-1.00pm, Bryant 302 (but

email me if coming on Wed.).4. Homework 11 due Wed. 29th Nov. by 11.59pm.5. Reading this week: Ch. 0-4.3, 5-17, 18, 4.46. Email me Astro-news, jokes, tunes, images: ast1002_tan-

l@lists.ufl.edu7. Printed class notes? Name tags?8. Final Exam: Tue., 5th December, 10.40am, in class (about 1/2 the

questions on material since midterm 2).You are NOT allowed calculators: questions will only require simple arithmetic. You will be given a list of all formulae used in the class (see next slide). Exam is multiple choice format on a scantron so bring a pencil. Bring your UF ID. In class review session on Thur. 1st Dec. - bring questions for discussion.

9. Extra review session - Monday 4pm-5.30pm - Bryant 217

All Formulae for FinalSpeed = distance / timeAngular size: θ = size / distanceKepler’s 3rd Law: P2 = a3

[ Newton’s version of Kepler’s 3rd: P2 ∝ a3/(m1+m2) ]Newton’s 2nd Law: F = m aNewton’s Law of Gravity: F ∝ m1 m2 / r2 Density = mass / volumeVolume of a sphere = (4/3)πr3

Surface area of sphere = 4πr2

Momentum = mass x velocityAngular momentum ∝ mass x rotation rate x size2

Frequency: f = 1/PeriodSpeed of wave (light) = frequency x wavelength: c = f λEnergy of Photon: E = h f Wien’s Law: λmax = 0.29cm / T(K) Parallax distance: d = 1/ p Flux: F = L / (4πd2)Stellar Luminosity: L = 4πr2 σT4

Doppler Shift: Δ λ / λ = v / cMain Sequence Luminosity: L ∝ M4

Stellar lifetime ∝ M / L ∝ M-3

Mass-Energy Equivalence E = m c2

Light Gathering Power = Area x Exposure timeResolving Power (angular resolution) = 0.25 λ (microns) / diameter (m)Orbits in Galaxies: Mgalaxy+ Msun ∝ a3/P2 ∝ a v2

Hubble’s Law: v = H0 dDrake Equation: Ntc = Rsf fwp Nsfl flb fil fts Lt

Key Concepts: Lecture 36: The Search for Planets and Life

Search for planets: Radial Velocity Search; Transit Technique; Microlensing Technique; Direct Detection

The Habitable Zone

Estimates from Drake Equation

SETI and Fermi’s Paradox

The Search for Planets Beyond the Solar System (=Extrasolar Planets = Exoplanets)

• Why is it hard to find planets?

1. Planets are faint (small; low temperature): Internal luminosity L = 4πr2 σT4

Reflected luminosity may be more important than internal

2. Habitable planetary systems will be close to a star, which is much brighter than the planet

Lsun compared to Learth

L = 4π r2 σ T4

Planet Search Techniques• Planet Searches: (Many

more extrasolar planets are now known than there are planets in our own solar system)1. Radial velocity search:

gravity of planet causes star to wobble. Look for the blue and redshift of spectral lines from the star• Most sensitive to

massive planets that are close to their stars

• detected >~500 planets to date by this method

Radial Velocity Search • Look for periodic Doppler shift due to wobble of star

caused by orbiting planet

The “Hot Jupiters”

• How are these planetary systems different from our own solar system?

• Why are we finding these kinds of systems?

They have massive planets very close to the star in the warm/hot region we expect terrestrial planets to form. Maybe these planets formed further from the star and migrated inwards. Often the orbits are eccentric.

The radial velocity search technique is most sensitive to massive planets that are close to their parent star: they produce a large wobbling of the star. (Recall Newton’s Law of Gravity: F ∝ m1m2/r2)Searches have only been in progress for ~20 years, so we have only had time to find planets with orbital periods shorter than this.

About 5% of surveyed stars have Hot Jupiters

Artist’s Impressionwww.extrasolar.net (John Whatmough)

• The glacier covered moon of 47 Ursae Majoris' planet. (note we do not know if the planet has a moon) Deep below the icy surface may exist an ocean of liquid water, and possibly, life.

• The second "Hot Jupiter" to be discovered, the planet orbiting 55 Cancri, like its cousin 51 Pegasi B, is awash in the glare of its nearby sun. On the night side of this seething world, we see super bolts of lightning arcing through the tumultuous atmosphere.

Nov 2005:

• www.exoplanets.org

• By end of 2005 about 160 planets found by radial velocity method.

• By 2014 we have found ~1000 planets in total by the radial velocity method.

2. Transit Search

• A planet passes in front of its star. We see a small dip in the brightness of the star.

• If we know the size of the star, we can then measure the size of the planet.

Depth of Transit Depends on Relative Sizes of Star and Planet

• Transit search technique measures the size of the planet (if we already know the size of the star)

Planetary Transits

First Planetary Transit Detected in 1999

• Star is called HD209458• Radius of planet is 1.3 times

Jupiter’s radius• Mass of planet (from radial

velocity technique) is 0.63 times Jupiter’s mass

• Why is the planet so big?

Probably because it is so close to its star, is strongly heated, and so is puffed up.

NASA’s Kepler telescope

Kepler has found >3000 planet candidates via the transit method.

Many multi-planet systems have been found, often with Earth to 10x Earth sizes in orbits within 0.3 AU.

The formation of these compact planetary systems, which are very different from our Solar System, is not yet understood.

Space telescope that stared continuously at ~160,000 stars towards constellation Cygnus

Hot Jupiters

Super Earths/Hot Neptunesfound by transits

Systems with Tightly-packed Inner Planets (STIPs)

Architecture of Kepler Multiplanets 3

Figure 1. Systems of three or more planets. Each line corresponds to one system, as labelled on the right side. Ordering is by theinnermost orbital period. Planet radii are to scale relative to one another, and are colored by decreasing size within each system: red,orange, green, light blue, dark blue, gray.

Fabricky et al. (2014)

3. The Microlensing Technique

Microlensing is when a star (the Lens star) passes in front of a background star (the Source star) and acts as a gravitational lens, bending the light to our telescopes (recall Einstein’s General Relativity). This causes the source star to appear to brighten and then fade over time (see fig. on right). If the lens star has a planet, this can cause an extra blip in the magnification. This method is good at finding small planets that are far from their star (in contrast to the radial velocity method). But it requires monitoring millions of source stars and generally the planets that are found are so far away from us they are difficult to study further with other techniques.

4. Direct Detection• Why is it hard to find planets?

1. Planets are faint (small; low temperature): Internal luminosity L = 4πr2 σT4

Reflected luminosity may be more important than internal

2. Habitable planetary systems will be close to a star, which is much brighter than the planet

HR 8799 planetary system, Nov. 2008

• Ntc = Rsf fwp Nsfl flb fil fts Lt• Ntc = number of technological civilizations now present in the Milky Way

• Rsf = rate of star formation over lifetime of the Galaxy ~ 10

• fwp = fraction of stars with planetary systems (>0.05) ~ 1 - 0.1

• Nsfl = average number of planets suitable for life

• flb = fraction of habitable planets where life arises

• fil = fraction of life-bearing planets where intelligence evolves

• fts = fraction of intelligent-life planets that develop technology

• Lt = average life time of a technological civilization

The Drake Equation

Number of Planets Suitable for Life: The Habitable Zone

• Need liquid water• Small Temperature

Range: 273-373K–Only planets with

certain distances from their star can maintain liquid water

–alternative heat sources• tidal forces• volcanoes

Habitable Zones around different stars

Fraction Where Life Arises

• Life on Earth arose very rapidly–flb ~ 1

• Anthropic Principle –we would only exist in a solar system where life

arose–Earth need not be representative–flb <<< 1

Fraction with Intelligent Life• Intelligent life arose on Earth despite many

catastrophic mass extinctions– once life forms, intelligence is inevitable– fil ~ 1

• Anthropic Principle - Earth must have intelligent life for us to ask this question– Earth is not representative!– fil <<< 1– Even on Earth it took several billion years for

intelligent life to develop.

Technological Civilizations?

• Advanced civilizations developed in several places on Earth–chances are likely that any one such civilization

would have developed communication technology• How long does this communication phase

last?–Very difficult to predict.–On Earth - only ~100 years so far!

• Ntc = Rsf fwp Nsfl flb fil fts Lt• Rsf = rate of star formation over life time of galaxy = 10 per year• fwp = fraction of stars with planetary systems = 1 - 0.1• Nsfl = Average number of planets suitable for life = ~0.2?• flb = fraction of habitable planets where life arises = 1 - 0.001 - ???• fil = fraction of life bearing planets where intelligence evolves = 1 - 0.1 - ??• fts = fraction of intelligent life planets that develop technology = 1 - 0.1 ?• Lt = average life time of a technological civilization = 109 - 100 years ?• Ntc = number of technological civilizations present in the Milky Way

• Ntc = 1010 - 10-4 civilizations

The Drake Equation

How Close is our Nearest Neighbor?• For our extremely optimistic value of 1010

communicative civilizations –average distance to nearest neighbor 6 light years

• For less optimistic values of ~ 1000 civilizations (this is still very optimistic)–average distance to nearest neighbor ~ 1000 ly

SETI

• Attempt to detect radio signals from extra-terrestrial civilizations

• Began in 1960s• Lost federal funding in early 1990s• Period of private funding - e.g. Paul Allen of

Microsoft, recently discontinued...• And restarted in 2015! Breakthrough Listen!

Jodie Foster in “Contact”

How many star systems have heard us?• We have been sending radio

signals into space for about 70 years.

• These have now traveled 70 light years in every direction.

• On average there is 1 star in every 4x4x4 ly3 =64 ly3

• The volume of the radio sphere is 4/3 πr3 = 1.4x106ly3

• So there are about 22,000 stars in this volume, that could have detected our radio/TV.

70 light years

4 light years

schematic diagram not to scale

Fermi’s Paradox• If there are a multitude of advanced extraterrestrial

civilizations in our Galaxy, the Milky Way, then, where are they? Why haven't we seen any traces of intelligent extraterrestrial life, such as probes, spacecraft or transmissions? e.g. an intelligent civilization should be able to spread out and colonize the entire Galaxy within tens of millions of years.

- Perhaps they are already here - studying us discretely (Zoo hypothesis)- Perhaps we are the only ones - formation and survival of intelligent life is difficult- Perhaps communication & interstellar travel are much more difficult than we imagine

Search for Earth-like Planets

• Problem –planets faint –stars bright

• Solution –Very precise optics–Telescopes in Space–Optical and infrared

wavelengths

www .jpl.nasa.gov

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ss

TPF: Terrestrial Planet Finder

HR 8799 planetary system, Nov. 2008

Searching for Earth-like PlanetsOur solar system as it would be seen by TPF

from a distance of 10 parsecs

Earth

By the year 2030 or so, we hope!

Search for Life - Oxygen

• Oxygen in planetary atmosphere is a signature of biological “life” processes

• Oxygen can be detected from the spectrum at mid-infrared wavelengths via OZONE

UFOs

• Einstein Special Relativity– nothing can travel faster than speed of light– severely limits interstellar travel and visitation– Probably would involve large scale colonization

expeditions that would take thousands of years• Nearest Neighbors?

– 6 (very very optimistic) to several thousand (still optimistic) light years away. But we do not really know.

• Most UFO sightings are explainable as astronomical objects such as Venus

Astro News

BreakthroughStarshot

Question

• You encounter a UFO and make contact with ET. You are only allowed to ask one question. What would that question be??

The Scientific Method - review

Propose theories to explain nature. The theory should make some prediction which has a chance to tested.

Test theories against experiments or observations

Reject theories that fail to explain the data

Keep those theories that work, but admit there is the possibility they are still wrong - there is always some uncertainty/doubt. Generally prefer the simplest theories that can explain the data.

Further Reading and Info• Astronomy News:

– www. space.com, www. spaceflightnow.com• US Astrophysics Policy for the next decade:

– http://sites.nationalacademies.org/bpa/BPA_049810• Astronomy at UF (www.astro.ufl.edu)

– Public lectures, telescope observing and Starry Night events– AST2037 Life in the Universe– AST3043 History of Astronomy– Consider a B.A. in Astronomy -> Law, Business, Medical

school, etc, or Science Education (UFTeach)– Consider a B.S. in Astrophysics -> as above, and/or career in

research