class events: week 12

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Class events: week 12Goals: learn about extrasolar planets

Methods of detectionPlanets observedTowards detecting lifeSolar system creation theoriesThe Rare Earth Hypothesis

Extra readings:http://en.wikipedia.org/wiki/Extrasolar_planet

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Challenges in detection

Visual detections of planets are difficult because their photons are swamped by the central star.

L /LEarth=1.5×109

L/LJupiter=4.1×108

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Direct observations

Imagery of the object itselfThis has been achieved using speckle interferometry or other modern methods for only a few planets.

Spectral data of the objectWhile brightness differences of suns and their planets are huge (i.e., 109× difference for Jupiter vs. our Sun in optical), they can be less overwhelming in infrared (i.e., a 105× difference in IR).

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Astrometric detections

Looking for tiny shifts in stellar position

Seeking planets because of their gravitational influence on the central star is possible, but difficult because of the mass difference.

With some algebra…

For Earth-Sun system, X* = 3×10-6 a.u., 450 km, 0.00065 R

For Jupiter-Sun system, X* = 1.0 R.

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The Doppler effect and radial velocityWaves emitted by approaching [receding] objects are shifted to shorter [longer] wavelengths. This is called the Doppler effect, with blueshifts and redshifts.

By analyzing the Dopplershifts of photons, the line-of-sight component of an object’s velocity can be measured.

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Radial velocity detections of planets

Look for Doppler shifts exhibited by the central star in a planetary system.

– Highly effective (pre-Kepler, the vast majority of exoplanets were found this way, including 51 Pegasi, the first star with an exoplanet discovered).

– Asymmetries in the stellar motions can indicate orbital parameters such as eccentricities, as in 70 Virginis.

– In some cases, even multiple planet systems can be analyzed.

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Doppler detections

HOWEVER…

Orbital tilts mean we only measure some, and not all, of the orbital velocity.

Therefore, we only measure a portion of the Doppler shift from the planet, and the star may be getting yanked about harder than we know.

This method only gives us a lower limit for the planets. (The value is distorted by cos θ.)

Fortunately, while we cannot correct a single planet’s mass for this effect, on average, it is not too bad for a sample of exoplanets:

Only a 33% chance of a planet being more than 2× the inferred mass;Only a 13% chance of a planet being more than 5× the inferred mass; Only a 6% chance of a planet being more than 10× the inferred mass;BUT a 0.6% chance of a planet being more than 100× the inferred mass!

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Transit detections

Looking for stellar eclipses...– This method is effective only if the orbital plane is closely

aligned with the Earth.– This alignment does not have to be as highly coincidental for

cases where the planet is very close to the star.– Jupiter would cause a 1.1% brightness drop for the Sun.– The Earth would cause a 0.008% drop for the Sun.

Many stars have brightness variations that exceed this. Therefore, the job is to look for highly periodic brightness changes.

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Stellar transits detected by Kepler

Kepler was launched in 2009.

10.5° square field of view

150,000 stars, every 30 minutes!

As of Feb 2014…– About 1800 planet confirmed candidates;– About 1800 planet confirmed candidates;– 23% Jupiter to super-Jupiters (6-22 REarth);

– 40% Neptune-sized (2-6 REarth);

– 26% super-Earth (1.25-2 REarth);

– 10% ≈ Earth-sized (R< 1.25 REarth).

– Most (76%) are Neptune-sized or smaller.– Many are within the habitable zone.

(More at http://exoplanet.eu)

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Gravitational lensing

OGLE-2005-BLG-390Lb: 5.5 MEarth, T~50 K, 2.1-4 a.u. from a (red dwarf?) star.Detected at a range of about 7000 parsecs!

General relativity shows us that gravity can bend beams of light.

One manifestation of this is to make stars wink brightly, as their light is focused towards us.

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Future missions

Transiting Exoplanet Survey Satellite (TESS)

500, 000 G-K stars.

Plans to focus on Earthlike planets

August 2017 SpaceX Falcon launch date

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The planetary zoo

Many of the planets detected are huge, and very close to their stars.

The most extreme of these are more massive than Jupiter, but are closer than about 0.05 a.u.

(Mercury is at 0.4 a.u.).

These are called hot Jupiters.

Smaller versions are calledhot Neptunes

Our detection methods wouldtend to preferentially detectthese planets.

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The planetary zoo

WASP-12b

An extreme Hot Jupiter.

1.4 MJupiter

1.74 RJupiter

1.09 day orbital period

Home star: G

Surface T: 2500K

It will be vaporized inabout 10 million years.

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The planetary zoo

HD 96167bAn “eccentric Jupiter.”

0.68 MJupiter

498 day orbital period

Home star: G5

e=0.710

7% of all systems haveeccentric Jupiters.

They are more commonthan hot Jupiters!

It is unlikely that other planets can share the system with an eccentric Jupiter!

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The planetary zoo

HD 189733bThe azure planet

1.16 MJupiter


Home star: K1-2 V

Despite being a hot Jupiter, its color has been

measured as being deep blue.

Spectroscopy has detected atmospheric

molecule information!

K, Na, CO2, H2O, O2, CH4

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The planetary zoo

Kepler-10b

One of the first rocky planets verified.

4.55 MEarth

1.39 REarth


T=2800 K

Home star: G

Kepler-10c

One of the biggest super-Earths known.

17 MEarth

2.35 REarth


T=584 K

Home star: G

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The planetary zoo

Gliese 1214b (super-Earth)

Based upon its mass and radius, estimates can be made about its composition and structure.

Its spectrum has been detected and isfeatureless—one explanation is that itsatmosphere is water-steamy.

Its overall composition may be 25% rock, 75% water.

6.36 MEarth

2.69 REarth


Home star: M

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The planetary zoo

Habitable super-Earth

Kepler-22b11-30 MEarth

2.4 REarth

(g=2-3gEarth)


Home star: G5

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The planetary zoo

Very Earthlike

Gliese 581d HD 85512b

~6.04 Mearth ~3.50 MEarth

66 day orbital period 54 day orbital period

Home star: M2.5 Home star: K5

Triple planet system

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The planetary zoo: Earthlike and in the habitable zone!

Kepler 186 f~1.13 Rearth Home star: M1

130 day orbital period Five-planet system

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The planetary zoo

Kepler-20e, KOI-961: The smallest planets detected so far.

Kepler-20e KOI-961 0.4-1.7 Mearth Sub-Earth?

0.87 REarth

6.1 day orbital period 0.45, 1.2, 1.9 day

Home star: G8M star

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The planetary zoo

Planets in binary/multiple star systems.

Kepler-16 (AB)b0.33 MJupiter

0.74 RJupiter


Home star: K, M

Alpha Cen Bb1.13 MEarth

3.23 day orbital periodHome star: K1

This case is one where the planet orbits a single star, which is in a multiple system with a G2 and M5 star.

The azure planet is in a similar double system

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The planetary zoo

HD 10180 - A planetary system around a G1V star

HD10180b(?) 1.4 MEarth 1.18d

HD10180c 13.1 MEarth 5.76d

HD10180d 11.8 MEarth 16.36d

HD10180e 25.1 Mearth 49.74d

HD10180f 23.9 MEarth 122.76d

HD10180g 21.4 MEarth 601.20d

HD10180h(?) 63.6 MEarth 2222.0d

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The planetary zoo

Gliese 667- A complicated system

Gliese 667A (K3V, 0.12 LSun) orbits Gliese 667B (K5V, 0.05 LSun) in 42 y

Gliese 667C (M1V, 0.014 LSun) orbits the pair in xx days

Gliese 667Cb 4-7 MEarth 7.2d

Gliese 667Ch(?) 1-3 MEarth ~17d

Gliese 667Cc 3-5 MEarth 28.1d (Habitable zone)

Gliese 667Cf 2-4 Mearth 39.1d (Habitable zone)

Gliese 667Ce 1-4 MEarth 62.3d (Habitable zone)

Gliese 667Cd 3-7 MEarth 91.6d

Gliese 667Cg(?) 3-8 MEarth 256d

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The planetary zoo

PSR J1719-1438b (the diamond planet)

Formerly a red giant star, and then a white dwarf in a binary. (Its companion already converted itself into a pulsar.)

The pulsar blew away nearly all the white dwarf star, and the remaining residual carbon-rich core is now considered a “diamond planet.”

1.02 MJupiter

0.4 RJupiter (4 REarth)

2.18 h orbital period

Home star: pulsar

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Some (soft) planetary stats

Estimates of planetary numbers still varies widely from team to team. However, all are suggesting that planets are common…

Analyses of Kepler data suggest that stars in the galaxy have, on average, 1.6 planets. Therefore, about 160 billion planets exist in the galaxy.

11 billion of these planets may orbit within the habitable zone of sunlike stars.

1.4—2.7% of all sunlike star systems are expected to have an Earthlike planet within the habitable zone.

Oversized planets, orbiting in the habitable zone, may have habitable moons!

Planets in unbound orbits may number in the trillions (1012)!

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Detecting exoplanetary life

The heat is on for detecting Earth-size planets in habitable zones...

If one is found, how could Earthbound scientists look for exoplanetary life?

Look for oxygen, methane, or other suspicious compounds in the atmosphere. So far, we have detected atmospheric K, Na, CO2, H2O, O2, CH4 in the azure planet and others.

Like the Martian meteorite ALH84001, however, evidence would have to be very, very strong.

Turning the tables…these lines of evidence are present in abundance in the Earth’s atmosphere. Curious alien astronomers that point their telescopes towards Earth would easily detect our signatures of life…

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Hot Jupiters and solar system theories

Recall our theory of solar system formation.

Hydrogen planets would not form close to the central star, because the proto-planetary disk would have been so hot that hydrogen, helium, and hydrogen-rich compounds would have been in gas from.

This is why we have terrestrial planets close to the Sun, and Jovian planets far from the Sun.

Hot Jupiters do not fit into our model of having terrestrial planets close to the star, and jovian planets far from the star.

Is our notion of planetary formation wrong?

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Hot Jupiters modify our solar system theories

If hot Jupiters did not form where they are seen today, it is possible their orbits shifted?

Density wave brakingGravitational effects from the planetary disk. This would work on planets that formed early, when the proto-planetary disk was still thick, and had not yet been dispelled by the stellar wind.

Jovian-jovian gravitational interactionsEncounters between planets could expel one, and send the other into an elliptical, near-star orbit.

Could terrestrial planets survive the inward migration of Jovian planets? It might be the case that planetary systems with hot Jupiters cannot have terrestrial planets in the habitable zone.

Modern thoughts on our solar system is that the planets were not always in their current locations. Solar systems change over time!

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Highly elliptical orbits

In our solar system, orbits are very nearly circular:

Mercury 0.206 Jupiter 0.048Venus 0.007 Saturn 0.056Earth 0.017 Uranus 0.046Mars 0.093 Neptune 0.010

We have also discovered that many exoplanets have very elliptical orbits (~50% have e > 0.2, ~17% have e > 0.5).

In some cases (~35%) these could result from additional, undetected planets confounding our interpretations of the data. Others (~40%) might be due to simple misinterpretations of the data.

Or…our planetary system is something of an oddity.

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Rare Earth hypothesis

“Life in the Universe” authors certainly seem to lean towards the notion of a universe filled with life. But what of the counter-hypothesis?

Rare Earth Hypothesis“Life, at least in an advanced multicellular form, is exceedingly rare in the Universe. The Earth may even be unique in this respect.”

Let us consider five factors that might make life rare.

1. The galactic habitability zone is smallFrequent supernovae set the inner limit of habitability.

Bennett & Shostak argue that such pulses of radiation might not be so bad; they may be shielded by the atmosphere, and might even encourage mutations that enhance evolution.

The rarity of elements more massive than helium sets the outer limit of habitability.

The difference (0.1% vs. 2%) does not prohibit the formation of rocky planets. A 12.7 BY old Jupiter has been found in M4!

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2. A Jupiter is necessaryPossibly, Jupiter was critical in expelling inner solar system comets to purgatory in the Oort cloud. Without this cleanup service, comets would continue to pelt the terrestrial planets, repeatedly sterilizing them.

(However, Jupiters and super-Jupiters have already been discovered in abundance, and so are not likely to be rare.)

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3. Having a large satelliteThe terrestrial planets are constantly being tugged and jostled gravitationally by the other planets. The tidal forces from our Moon overwhelms these other tugs, and keep our axial tilt more or less stable at 23.5º.

On the other hand, Mercury, Venus, and Mars do not have such a large moon. This could contribute to very large climate variations on an otherwise habitable world.

Obtaining a massive moon may be both critical for life, and highly unlikely!

Maybe large moons are not highly unlikely, some of the Kuiper Belt Objects have them.

And is climate stability really important? Recall that the Cambrian explosion of life diversity may have resulted from a massive climate transition from a snowball Earth phase to a hothouse Earth phase.

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4. Having plate tectonicsThe CO2 cycle is a stabilizing influence for our climate. This requires active plate tectonics.

It might be the case that having plate tectonics is rare. For example, we do not see it well developed in Mercury, Venus, or Mars.

Since Mercury and Mars are both small, we should not be surprised at the lack of plate tectonics—but what about Venus?

Venus’ enormous greenhouse effect may be to blame for the lack of plate tectonics—the water was cooked out of the crust.

There is no reason to insist that an Earth-sized planet in a habitable zone must have a runaway greenhouse effect (the Earth is proof of this).

If runaway greenhouses were the norm, why did the Earth dodge this bullet?

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5. Having an ocean, but not too much You can argue that, as a technological civilization, we are the results of an amphibious pattern of evolution:

– Life on Earth may have developed around undersea hydrothermal vents.

– In order to develop our necessary technological skills such as mastery of fire, our aquatic predecessors had to evolve into land-based life forms.

Therefore, in order to develop an advanced, technologically adept civilization, a planet must have adequate water, but not so much that continents do not form. This might be a delicate and improbable balance.

Opponents to this argue that aquatic civilizations may very well exist, and that the argument is based in a prejudiced perspective. Furthermore, the details of the land-ocean balance may not be very critical.

class events: week 12

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