DEPARTMENT OF PHYSICS AND ASTRONOMY

3677 Life in the Universe: Extra-solar planets

Dr. Matt Burleigh

www.star.le.ac.uk/mrb1/lectures.html

Course outline

• • Lecture 1 – Definition of a planet – A little history – Pulsar planets – Doppler “wobble” (radial velocity) technique Lecture 2 – Transiting planets – Transit search projects – Detecting the atmospheres of transiting planets: secondary eclipses & transmission spectroscopy – Transit timing variations Dr. Matt Burleigh 3677: Life in the Universe

Course outline

• • Lecture 3 – Microlensing – Direct Imaging – Other methods: astrometry, eclipse timing – Planets around evolved stars Lecture 4 – Statistics: mass and orbital distributions, incidence of solar systems, etc.

– Hot Jupiters – Super-Earths – Planetary formation – Planetary atmospheres – The host stars Dr. Matt Burleigh 3677: Life in the Universe

Course outline

•

Lecture 5

– The quest for an Earth-like planet – Habitable zones – Results from the Kepler mission • How common are rocky planets?

• Amazing solar systems – Biomarkers – Future telescopes and space missions Dr. Matt Burleigh 3677: Life in the Universe

Useful web sites

• • • • • • Extra-solar planets encyclopaedia: exoplanets.eu

Exoplanet Data Explorer (California Planet Survey): exoplanets.org

NASA exoplanet archive: exoplanetarchive.ipac.caltech.edu

Planet hunters (Zooniverse): www.planethunters.org

Kepler mission: kepler.nasa.gov

Next Generation Transit Survey: www.ngtransits.org

Dr. Matt Burleigh 3677: Life in the Universe

Useful books

• • • • • • Extrasolar planets & Astrobiology:

Caleb A. Scharf

Extrasolar planets: the search for new worlds:

Stuart Clark

Transiting Exoplanets:

Carole A. Haswell

The Exoplanet Handbook:

Michael Perryman

An Introduction to Astrobiology:

Iain Gilmore & Mark Sephton

Life in the Universe:

Bennett & Shostak

Dr. Matt Burleigh 3677: Life in the Universe

Useful numbers

• • • • R Sun = 6.995x10

8 m R jup = 6.9961x10

7 m ~ 0.1R

Sun R nep = 2.4622x10

7 m ~ 4R earth R earth = 6.371x10

6 m ~ 0.1R

jup ~ 0.01R

Sun • • • • M Sun = 1.989x10

30 kg M jup = 1.898x10

27 kg ~ 0.001M

Sun = 317.8M

earth M nep = 1.02x10

26 kg ~ 5x10 -5 M Sun M earth = 5.97x10

24 kg = 3x10 -6 M Sun ~ 0.05M

jup = 17.15M

earth = 3.14x10

-3 M jup • • 1AU = 1.496x10

1 day = 86400s 11 m Dr. Matt Burleigh 3677: Life in the Universe

What is a planet?

•

International Astronomical Union definition –

– An object orbiting a star – Too small for dueterium fusion to occur • Less than 13 times the mass of Jupiter – Formation mechanism?

• Forms from a circumstellar disk of dust and gas around a young star – Lower mass limit – IAU decided that Pluto should be downgraded!

Dr. Matt Burleigh 3677: Life in the Universe

What is a planet?

Above, left to right: limb of Sun, late M (red) dwarf, L brown dwarf, T brown dwarf, Jupiter.

The coolest stars, old brown dwarfs and gas giant planets have the same radii!

Dr. Matt Burleigh 3677: Life in the Universe

A brief history of extra-solar planets

• • • • •

16th century:

the Italian philosopher Giordano Bruno said that the fixed stars are really suns like our own, with planets going round them

19 th Century:

anomalies in the binary star 70 Oph could be explained by an unseen planet, but later disproved astronomers believed orbital 1950s & 60s: Barnard Peter van de Kamp concluded that irregularities in the high proper motion of nearby ’s Star were caused by a planet. Sadly, this too turned out to be erroneous.

late 1980s:

Canadian Gordon Walker found tentative evidence for exoplanets using radial velocity method: but not confirmed until 2000s!

1991:

Andrew Lyne & Setnam Shemar at Jodrell Bank claimed to have discovered a pulsar planet in orbit around PSR 1829-10, using pulsar timing variations. They withdrew the claim later that year due to an error in their calculations.

New York Times 16 th April 1963

Dr. Matt Burleigh 3677: Life in the Universe

A brief history of extra-solar planets

• • • 1991 Radio astronomers Alex Wolszczan & Dale Frail discovered planets around a pulsar PSR1257+12 – Variations in arrival times of pulses suggests presence of three or more planets – Planets probably formed from debris left after supernova explosion 1995 Planet found around nearby Sun-like star 51 Peg by Swiss astronomers Michel Mayor & Didier Queloz using the “ Doppler Wobble ” method – Most successful detection method by far, but other methods like transits are now very successful >1700 exoplanets

confirmed

to date by all methods – Kepler has several thousand more

candidates

Dr. Matt Burleigh 3677: Life in the Universe

Blue : radial velocity, Green : transiting, Red : microlensing, Orange : direct imaging, Yellow: pulsar timing Dr. Matt Burleigh 3677: Life in the Universe

Dr. Matt Burleigh 3677: Life in the Universe

Dr. Matt Burleigh 3677: Life in the Universe

Pulsar planets

• • • Pulsars are neutron stars that emit radio pulses every ~second as they spin – More stable and accurate than an atomic clock If a planet accompanies the pulsar, then the pulsar will orbit the centre of mass of the system – The pulses will then arrive earlier or later than expected Radio observations have found a dozen or so such in Puerto Rico were the first confirmed exoplanets “pulsar planets” – Wolszczan & Frail’s discovery of PSR1257+12’s planets in 1991 at Arecibo – PSR1257+12’s planets are all ~Earth mass or smaller Dr. Matt Burleigh 3677: Life in the Universe

Pulsar planets

• Pulsars are created when a massive star (>8M sun – Their original planetary systems will not survive from supernova debris high energy radiation from pulsar ) explodes as a supernova – Radio-detected planets thought to have formed – Planets will be bathed in – no chance of life!

Dr. Matt Burleigh 3677: Life in the Universe

Planet Hunting: The Radial Velocity Technique (

“

Doppler Wobble

”

)

• Star + planet orbit common centre of gravity • As star moves towards observer, wavelength of light shortens (blue-shifted) • Light red-shifted as star moves away

517 planets detected by Doppler Wobble and many more transiting planets confirmed by this method

Dr. Matt Burleigh 3677: Life in the Universe

Measuring Stellar Doppler shifts

• • Method: – Observe star ’ s spectrum through a cell of iodine gas – Iodine superimposes many lines on star ’ s spectrum – Measure wavelength (or velocity) of star ’ s lines relative to the iodine Measure:  Dl / l e = ( l 0 -l e ) / l e = v r / c l o = observed wavelength, l e =emitted wavelength Dr. Matt Burleigh 3677: Life in the Universe

M

* from spectral type Dr. Matt Burleigh 3677: Life in the Universe

Doppler Wobble Method

• • Since measure K (= v * sin i), not v * directly, only know mass in terms of the orbital inclination i Therefore only know the planet ’ s

minimum

mass,

M

– If i=90 o (eclipsing or

transiting

) then know mass exactly sin

i

Orbital plane i=90 0 Orbital plane i 0

Dr. Matt Burleigh 3677: Life in the Universe

Example: 51 Peg

•

P= 4.15days = 4.15x86400s = 3.5856x10

5

s

•

G5V star, M

*

=1.11M

sun

= 1.11x1.989x10

30

kg = 2.21x10

30

kg

•

Find r = 0.052AU, M

pl

v

pl

=1.37x10

5

ms

-1

sin

i =

0.45M

jup Dr. Matt Burleigh 3677: Life in the Universe

Above: eccentric orbit (e=0.93) Top right: 55 Cancri multiple-planet system (4, maybe 5 planets) Bottom right: 3 planet HD37124 system Dr. Matt Burleigh 3677: Life in the Universe

HARPS radial velocity spectrograph

• Built by Geneva Observatory • First installed on ESO 3.6m at La Silla, Chile in 2002/3 • Has found over 130 planets • Precision 30cm/s – 1m/s • Simultaneously observes star and a reference Thorium lamp through two separate fibres • Highly stable optical bench, housed in sealed, thermally stable room • Second HARPS installed on Italian Galileo telescope on la Palma 2012 Dr. Matt Burleigh 3677: Life in the Universe

Doppler Wobble Method

• • Precision of current surveys routinely <1m/s – Jupiter causes Sun ’ s velocity to vary by 12.5m/s – All nearby, bright Sun-like stars are good targets • Lots of lines in spectra, relatively inactive – Smallest planet found by this method is

~1

M earth : Alpha Cen Bb – nearest star system to us!

– Most are Neptune size and larger Length of surveys limits distances planets have been found from stars – Earliest surveys started 1988 – Jupiter (5AU from Sun) takes 12 yrs to orbit Sun – Saturn takes 30 years • Would be strongly hinted at but not yet completed one orbit since surveys began – Do not see planet

directly

Dr. Matt Burleigh 3677: Life in the Universe

Alpha Cen Bb

• • Alpha Cen system is the nearest star system to us • Alpha Cen B has been monitored by radial velocity method • Very recent discovery of a rocky planet: • Minimum mass 1.1xEarth

• Period 3.2 days •

Dumusque et al. 2012, Nature Thought: if Alpha Cen B has a rocky planet, do most stars have rocky planets?

Dr. Matt Burleigh 3677: Life in the Universe