Chapter 10: Other Planetary Systems: The New Science of Distant Worlds

Our goals for learning:• How do we detect planets around other stars?• How successful are these methods?

How do we detect planets around other stars?

Hunting For Planetsl Have to use light coming to us- no

interstellar exploration :(

l Direct: Pictures or spectra of the planets themselves

l Indirect: Measuring the effects of planets on the properties of their parent stars.

(Stellar wobble, Doppler shift effects, brightness changes during transits/eclipses)

Gravitational Tugging-Stellar Wobble

l Sun and Jupiter orbit around their common center of mass

l Sun therefore wobbles around that center of mass with same period as Jupiter

Gravitational Tugging-Stellar Wandering

l Sun’s motion around solar system’s center of mass depends on tugs from all the planets

l Astronomers around other stars that measured this motion could determine masses and orbits of all the planets

Stellar Wandering (Astrometrics)

l We can detect planets by measuring the change in a star’s position on sky

l However, these tiny motions are very difficult to measure (~0.001 arcsecond)

Doppler or 'Radial Velocity' Technique

l Measuring a star’s Doppler shift can tell us its motion toward and away from us

l Current techniques can measure motions as small as 1 m/s (walking speed!)

l The first-ever planet was discovered in 1995 by the Doppler technique

Other Planet-Hunting Strategies

• Gravitational Lensing: Mass bends light in a special way when a star with planets passes in front of another star.

• Features in Dust Disks: Gaps, waves, or ripples in disks of dusty gas around stars can indicate presence of planets.

Transits and Eclipses

l A transit is when a planet crosses in front of a star

l The planet blocks out some of the star's light (an eclipse) reducing the star’s apparent brightness and tells us planet’s radius

Kepler: Mission to Measure Transits

• NASA's Kepler mission was launched in 2008 to begin looking for transiting planets.

• It is designed to measure the 0.008% decline in brightness when an Earth-mass planet eclipses a Sun-like star

Kepler: Mission to Measure Transits

• Observe a tiny patch of sky near Cygnus

• Onboard photometer constantly scanned 100,000 stars looking for the dimming effect

How Successful Are These Methods?

• From Earth, Doppler (radial-velocity) technique is most effective

• Astrometrics takes too long• Transits extremely difficult from Earth. Kepler

mission was made to be sensitive enough to detect small dimming effects.

• Amazing results – over 4000 possible planets!

What have we learned?• How do we detect planets around other

stars?–Direct starlight is billions of times brighter

than starlight reflected from planets making imaging difficult.

–A star’s periodic motion (detected through Doppler shifts) tells us about its planets

–Transiting planets periodically reduce a star’s brightness

What have we learned?• How successful are these methods?

–Doppler shift (aka raidal velocity technique) detected the most planets initially

–The Kepler telescope is sensitive enough to detect faint dimming of starlight due to a planet's transit

–Kepler has detected 1000s of possible planets including several multi-planet systems

Our goals for learning:• What properties of extrasolar planets can we

measure?• How do extrasolar planets compare with planets

in our solar system?• Do we need to modify our theory of solar

system formation?

Measurable Properties

• Orbital Period, Distance, Shape – directly measure from motions

• Planet Mass, Size, and Density – calculate from motion measurements

• Composition – measure from spectra under the right conditions

Measuring Mass: Newton&Kepler's Laws

p = orbital perioda=average orbital distance (between centers)(M1 + M2) = sum of object masses

The result:• The masses of any orbiting bodies can be calculated

from the size or period of their orbit (measureable quantities!)

p2=4p2

GM1+M2a3OR M1+M2=

4p2

Ga3

p2

highlighted.

l The mass of binary stars and extra-solar planet properties are derived this way.

l We know the mass of stars based on their energy-output and other behaviors

l The mass of the invisible planet(s) can then be calculated

Measuring Mass• We cannot measure an exact mass for a planet

without knowing the tilt of its orbit, because Doppler shift tells us only the velocity toward or away from us.

Doppler data can only give us lower limits on masses.

Planet shape, size -transit method only• Planet passes in front of its star -shape and size

become visible• Sometimes starlight passing through the planet's

atmosphere can be measured• Multiple planets can

be detected too -star systems!

E.g. Kepler-11 systemThe periods and sizes of Kepler 11's 6 known

planets can be determined using transit data.

Calculating Density

• Using mass, determined using the Doppler technique, and size, determined using the transit technique, bulk density can be calculated.

What have we learned?

• What properties of extrasolar planets can we measure?

– Orbital properties, such as period, distance, and shape.

– Planetary properties, such as mass and size.– Atmospheric properties, such as temperature

and composition.

How do extrasolar planets compare with planets in our solar system?

How do extrasolar planets compare with planets in our solar system?

• Exoplanet results strongly depend on which techniques used to find them

• Gravity tugging techniques favor big, swift planets.

• Only transits can detect the smaller planets. Mercury-size and smaller undetectable with current technology.

First Extrasolar Planet• Doppler shifts of star 51

Pegasi indirectly reveal a planet with 4-day orbital period

• Short period means small orbital distance

• First extrasolar planet to be discovered (1995)

Surprising Characteristics• Some extrasolar planets have highly elliptical

orbits

• Some massive planets orbit very close to their stars: “Hot Jupiters”

– Gas Giants that have smaller orbits than Mercury.

• Kepler has discovered a huge range of sizes, including Earth-sized.

Hot Jupiters

Kepler Mission

Some Earth-sized worlds detected in the habitable zones of stars!

(more in the hot Venus-zone)

Caveats

Habitable Zone ≠ Habitable Planet– A planet may be in a star's liquid water zone, this

does not mean that the planet must have water or life.

Earth-sized planet ≠ Earth-like conditions– A planet close to Earth in size may still be very

different in behavior and conditions (e.g Venus)

• Identifying these worlds is vital for target selection for the next generation of more-sensitive telescopes. (TESS, James Webb Telescope)

Sol System vs.Multi-planet Kepler Systems

Sol System vs.Multi-planet Kepler Systems

Our Solar System is Unusual

• Kepler transit results show planet frequency increases towards smaller (terrestrial) size.

• Recent work show that the transition to a H/He dominated atmosphere occurs around 1.5 Earth radii.

• Lots of worlds are 2-3 times size of Earth, yet our solar system does not have this size.

• A jump in planet-size:Terrestrial⇾Gas Giant found in our solar system is not normal.

Do we need to modify our theory of solar system formation?

• The nebular theory predicts that massive Jupiter-like planets should not form inside the frost line (at << 5 AU).

• Hot Jupiters and Super-Earths mean that our solar Nebula theory must be revised.

• Planetary migration or gravitational encounters may explain hot Jupiters.

Planet Migration

• A young planet's motion can create waves in a planet-forming disk.

• Models show that matter in these waves can tug on a planet, causing its orbit to migrate inward.

• Gravity assists and resonances can also have an effect

What have we learned?• How do extra-solar planets compare with the

planets in our solar system?–Earth-sized worlds are common–Many large-mass planets orbit close to their

stars

–Multi-planet systems exist and may be common. None are like us.

–Work is very preliminary, more sensitive telescopes needed to fully map discovered systems.

What have we learned?• Do we need to modify our theory of solar system

formation?

– Original nebular theory cannot account for the existence of hot Jupiters.

– Planetary migration or gravitational encounters may explain how Jupiter-like planets moved inward.