Transcript Searching for Planets around OTHER suns

Searching for planets around
OTHER stars…
BETA Pictoris (1984) – the first hint at
Stars with Planetary Disks?
BETA Pictoris (2006) – a solar system
in the making?
BETA Pictoris – now confirmed with a
Jupiter-sized planet
ESA image
from 2012:
Dust is like
that in our
solar
system!
Inferring planets that
aren’t seen!
Detecting Planets around OTHER stars!
Orbiting
Telescopes:
KEPLER
COROT
Hubble
Ground
Observations
WASP
Why are extrasolar planets
difficult to detect?
• Sun-like stars are billions
of times brighter than any
light reflected from planets.
• June 2016 – direct image of
a planet orbiting another
star (a LONG way away!)
• A 27,000-year orbit!
Why are extrasolar planets
difficult to detect?
• Planets are EXTREMELY close to their stars,
relative to distance from us to the star.
– Like being in San Francisco & trying
to see a pinhead 15 meters from
a grapefruit …
– … in Washington, D.C.
So given how bright stars are, how CAN we
detect planets around other stars?
Planet Detection
• Indirect: Look at the star…
– Measurements of stellar properties revealing
effects of the star’s orbiting planets
• Direct: Look for the planet
– Pictures or spectra of the planets themselves
Indirect Detection through Observation
• Stars hold onto planets
with gravity, but…
• Planets tug on their
stars, too!
• We can see the stars
move in space over
time…
• but they don’t move much!
Example from our Solar System:
Indirect Detection through Astrometry
• Sun & Jupiter orbit
around common
center of mass.
• Sun wobbles around
center of mass with
same period as
Jupiter (~12 years)
Astrometric detection
• We can detect
planets by measuring
change in star's
position on sky.
• However, tiny motions
are very difficult to
measure
(~ 0.001 arcsecond).
Combined Gravitational Tugs
• Sun's motion around
solar system's center of
mass depends on tugs
from all planets.
• Astronomers around
other stars measuring
this motion could
determine masses and
orbits of all planets!
Indirect Detection through Gravitation
• A faster way to detect presence of planets
• Use the stars “wobble” revealed through spectra
• Angle of solar system matters!
Doppler Technique
• Measuring a star's
Doppler shift can tell
us its motion toward &
away from us.
• Current techniques
can measure motions
as small as 1 m/s
(walking speed!).
1st Extrasolar Planet Detected (1995)
• The planet around 51 Pegasi has a mass similar
to Jupiter’s, despite its small orbital distance.
First Extrasolar Planet
• Doppler shifts of the star
51 Pegasi indirectly
revealed a planet with
4-day orbital period.
• Mass ~ ½ of Jupiter!
• But very short period
means that planet has
small orbit – not where
ice could form!
Another Indirect Method:
Transits and Eclipses
• A transit is when a planet crosses in front of a star.
Another Indirect Method:
Transits and Eclipses
• A transit is when a planet crosses in front of a star.
Another Indirect Method:
Transits and Eclipses
• Reduces star's apparent brightness & tells radius.
• Sometimes an eclipse can also be detected
Kepler
• NASA's Kepler mission
launched in 2008 to
begin looking for
transiting planets.
• Designed to measure
0.008% decline in
brightness when an
Earth-mass planet
eclipses a Sun-like star.
Kepler
Can you see a gnat flying in front of a car headlight
Planet Detection Challenges
Indirect Planet Detection Challenges
– Astrometry:
• takes lots of TIME
– Doppler Shift:
• detects LARGE planets NEAR to stars
– Transits:
• needs LUCK, & detects lots of “False Positives”
Doppler Method Challenges
• Most detected
planets have orbits
smaller than
Jupiter’s.
• Planets at greater
distances are
harder to detect
with Doppler
technique.
Transit Method Challenges
• Lots of things cause starlight to change other than a
planet’s transit!
– Variable stars
– Eclipsing binaries
– Brown dwarfs
• 2015 paper: up to 50% of Kepler’s finds are “false
positives?
• 2016 Study: ~ 30% confirmed as planets
The Process of Science
• Observation: TMR-1
“detected” in 1998
The Process of Science
• Observation: TMR-1
“detected” in 1998
• Hypothesis: It is a planet,
connected by a disk to
a double (binary star)
The Process of Science
• Observation: TMR-1
“detected” in 1998
• Hypothesis: It is a planet ?
• Critical Tests: Spectra
The Process of Science
• Observation: TMR-1
“detected” in 1998
• Hypothesis: It is a planet ?
• Critical Tests: Spectra
• Result: A background star!
The Process of Science
• We can be fooled!
• “Brown Dwarfs” exist
• “Super-Jupiters”
Still LOTS of
planets
detected in 5+
years by
Kepler
Orbits & sizes of extrasolar planets from
Kepler
• Results from Kepler through 2015 indicate that
planets are common, and small planets greatly
outnumber large planets!
Masses &
sizes of
extrasolar
planets
Direct Detection?
Direct Detection?
How do extrasolar planets compare
with our solar system?
Upsilon Andromeda has multiple planets!
Do we need to modify our theory of solar
system formation?
Do we need to modify our theory of solar
system formation?
Revisiting the Nebular Theory
• The nebular theory predicts that massive
Jupiter-like planets should not form inside the
frost line (at << 5 AU).
• The discovery of hot Jupiters has forced
reexamination of nebular theory.
• Planetary migration or gravitational encounters
may explain hot Jupiters.
Planetary Migration
• A young planet’s
motion can create
waves in a planetforming disk.
• Models show that
matter in these
waves can tug on a
planet, causing its
orbit to migrate
inward.
Hot Jupiters
Gravitational Encounters
• Close gravitational encounters between two
massive planets can eject one planet while
flinging the other into a highly elliptical orbit.
• Multiple close encounters with smaller
planetesimals can also cause inward migration.
Modifying the Nebular Theory
• Observations of extrasolar planets have shown
that the nebular theory was incomplete.
• Effects like planet migration and gravitational
encounters might be more important than
previously thought.
Are planetary systems like ours common?
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.
• Are planetary systems like ours common?
– The answer is coming soon…
Planets: Common or Rare?
• One in ten stars examined so far have turned
out to have planets.
• The others may still have smaller (Earth-sized)
planets that cannot be detected using current
techniques.
What properties of extrasolar planets can
we measure?
Calculating density
• Using mass,
determined using
Doppler
technique, & size,
determined using
transit technique,
density can be
calculated.
Other Planet-Hunting Strategies
• Gravitational Lensing: Mass bends light in a
special way when a star with planets passes in
front of another star.
Other Planet-Hunting Strategies
• Features in Dust Disks: Gaps, waves, or
ripples in disks of dusty gas around stars can
indicate presence of planets.