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Chapter 12. Extrasolar Systems

Extrasolar planet discovery:

- Pulsar planets- Wobble method (radial velocity)

- Transit (occultation, eclipse) method- Examples and statistics

ASTA01 @ UTSC – Lecture 17

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Observations by Hubble Space Telescope (NICMOS near-IR camera).

Age ~ 5 Myr, a transitional disk

Gap-opening PLANET ?So far out?

Only if migrated outward R_gap~350AdR ~ 0.1 R_gap

HD 1415969

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HD 14169A disk gap confirmed by new observations (HST/ACS)

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Alpha Pisces Austrini (α PsA)

Fomalhaut

A disk of a bright

southern star

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Planets Orbiting Other Stars

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Extrasolar Planets

Extrasolar Planets

• Think of someone walking a poorly trained dog on a leash.• The dog runs around pulling on the leash.• Even if it were an invisible dog, you could plot its

path by watching how its owner was jerked back and forth around the

Center of Mass

Extrasolar Planets

• In the same way, astronomers can detect a planet orbiting another star – by watching how the star moves as the planet tugs on it.

Extrasolar Planets

How the star moves is revealed by either:• It’s sinusoidal motion on a sky (astrometric

detection), or• How its light changes frequency due to te

Doppler effect (radial velocity detection), or• If it sends pulses as a pulsar, then by the

time delay of pulse arrival times

• When the star approaches us, we see lower frequency of pulses, or of electromagnetic waves

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Extrasolar Planets: discovery in 1992

• [In 1988, Canadian astronomers Bruce Campbell, G.A.H. Walker, and S. Yang discovered an extrasolar planet orbiting a binary star system, but their discovery was not confirmed until 2002.]

• The first 3 confirmed extrasolar planets were discovered around a pulsar by the Polish astronomer Alex Wolszczan (b. 1946)

[read: Volsh-chan] in 1992• He studied & worked in Toruń, the

city of Copernicus, but discovered planets with the Arecibo radio telescope in Puerto Rico, with his coworker D. Frail.

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Extrasolar Planets: discovery

• Pulsar’s name is PSR 1257+12• It formed in a supernova explosion and has 3 ms period of

rotation• 4 Planets have masses:

0.02, 4.3, 3.9, 0.0004 ME

distances 0.19, 0.36, 0.46, 2.6 AU

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Chart of three PSR 1257+12 planets & inner solar system planets

Extrasolar Planets: 51 Peg

• The first planet orbiting a sunlike star was discovered in 1995 around the star 51 Pegasi.

• As the planet circles the star, the star wobbles slightly.• The very small motions of

the star are detectable as Doppler shifts in the star’s spectrum.• This is the same

technique used to study spectroscopic binary stars.

Michel Mayor (b.1942), Switzerland

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Extrasolar Planets: 51 Peg

• From the motion of the star and estimates of the star’s mass, astronomers can deduce that the 51 Peg b planet has half the mass of Jupiter and orbits only 0.05 AU from the star ( << sun-Mercury distance)• Half the mass of Jupiter amounts to 160 Earth

masses. A large planet, larger than Saturn.

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Extrasolar Planets

• Astronomers were not surprised by the announcement that a planet orbits 51 Peg

• For years, they had assumed that many stars had planets

• Nevertheless, some of them (Canadian David Grey) greeted the discovery with skepticism

• That skepticism led to careful tests of the data and further observations that confirmed the discovery

Extrasolar Planets

• Over 500 planets have been discovered in this way – including at least three planets orbiting the star Upsilon Andromedae, and five orbiting 55 Cancri – true planetary system.• More than 40 such

multiple-planet systems have been found.

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Extrasolar Planets

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Extrasolar Planets

• The Spitzer Infrared Space Telescope has detected two planets when they passed behind their stars.• These planets are hot and emit significant

infrared radiation.• As they orbit their parent stars, astronomers detect

variation in the amount of infrared from the system.

Measurements reveal that they have Jupiter-like diameters as well as masses. So, astronomers

conclude they have Jovian densities and compositions.

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Extrasolar Planets

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Extrasolar Planets

• Planets known so far

2012

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Extrasolar Systems: the first images, HR 8799

• Actually getting an image of a planet orbiting another star is about as easy as photographing a bug crawling on the bulb of a searchlight miles away.• Planets are small and dim and get lost in the

glare of the stars they orbit.

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Beta Pictoris giant planet

Extrasolar Planets

• In 2007, astronomers discovered what could be low-mass Earth-like planets orbiting a red dwarf star named Gliese 581 located a mere 20.3 light-years away. • In 2011, a team of

scientists in France confirmed that at least one of the planets could have an atmosphere and oceans, and support Earth-like life.

Extrasolar Planets: Gliese 581

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Extrasolar Planets: Kepler satellite observatory

• The main aim of the Kepler mission is to find Earth like planets in habitable zones around other stars.• For the first time in the history of our search

for the worlds that resemble our own, we have the technical capabilities to see small rocky planets.

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Extrasolar Planets

• The sizes of the newly discovered planets range from 1.5 times the size of Earth to large Jupiter-sized worlds.• Spectral analyses of trails of smaller planets

show traces of silicates (building blocks of rocks), ice, and water. • The Spitzer infrared telescope, which prior to the

Kepler mission discovered numerous large, hot, Jupiter-like planets around their stars, is being used to confirm the Kepler telescope findings.

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Extrasolar Planets: Kepler 11 system

• In the few first months of the mission more than 1200 planet candidates were detected, many of them multiple planetary systems. • One such system is Kepler 11, six tightly packed

planets located 2000 light-years from Earth; planets range from 2.5 to 4.5 times Earth’s size.

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Kepler 11 system

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2011

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Extrasolar Planets

• Some unusual arrangements of planets have been discovered by the Kepler mission.• Notably, astronomers were shocked to see

two planets sharing an orbit – the planets were arranged in the exact angular distance that theoretically allows for such an arrangement.• Seeing such variety of possibilities allows us to

explore and test many hypotheses about our own solar system’s origin and formation.

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Extrasolar Planets

• The discovery of extrasolar planets gives astronomers added confidence in the solar nebula theory. • The theory predicts that

planets are common.• Astronomers are finding them

orbiting many stars.