DEPARTMENT OF PHYSICS AND ASTRONOMY

Life in the Universe: Extra-solar planets

Dr. Matt Burleigh

www.star.le.ac.uk/~mbu

3677 Timetable

• • •

Today, and next Friday: MB Extrasolar planets Then Derek Raine After Xmas: Mark Sims (Life in the solar system)

Dr. Matt Burleigh 3677: Life in the Universe

Dr. Matt Burleigh 3677: Life in the Universe

Contents

• •

Methods for detection

– Doppler “wobble” – Transits – Microlensing – Direct Imaging

Characterisation

– Statistics – Implications for formation scenarios Dr. Matt Burleigh 3677: Life in the Universe

Useful reading / web sites

• • • • • Nature, Vol. 419, p. 355 (26 September 2002) Extra-solar planets encyclopaedia California & Carnegie Planets Search How stuff works planet-hunting page – Includes lots of animations & graphics JPL planet finding page – Look at the science & multimedia gallery pages Dr. Matt Burleigh 3677: Life in the Universe

What is a planet?

•

International Astronomical Union definition –

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

• Forms from a circumstellar disk – Lower mass limit – IAU decided last year that Pluto should be downgraded!

Dr. Matt Burleigh 3677: Life in the Universe

A brief history of exoplanets

• • 1991 Wolszczan & 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 Planets found around nearby Sun-like star 51 Peg by Doppler “wobble” method – Most successful detection method by far – 265 exoplanets found to date Dr. Matt Burleigh 3677: Life in the Universe

Radial Velocity Technique (Doppler “Wobble”)

•

Star + planet orbit common centre of gravity

•

As star moves towards observer, wavelength of light shortens (is blue shifted)

•

Light red-shifted as star moves away

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 Dr. Matt Burleigh 3677: Life in the Universe

•

Measuring Stellar Doppler shifts

Method:

– Measure wavelength (or velocity) of star’s lines relative to the iodine  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

• N.B. M * comes from the spectral type Dr. Matt Burleigh 3677: Life in the Universe

Doppler Wobble Method: Summary

• Precision of current surveys is now 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 • Limited to gas planets and larger – Note recently discovered “hot Neptunes” (>14M Earth ) – Not yet suitable for Earth-like planets • Length of surveys limits distances planets have been found from stars – Earliest surveys started 1989 – Jupiter (5AU from Sun) takes 12 yrs to orbit Sun – Saturn takes 30 years • Would remain undetected • Do not see planet

directly

Dr. Matt Burleigh 3677: Life in the Universe

Doppler Wobble Method: Summary

• • 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 – If i=90 o (eclipsing or

transiting

) then know mass exactly

Orbital plane i=90 0 Orbital plane i 0

Dr. Matt Burleigh 3677: Life in the Universe

Transits

•

Planets observed at inclinations near 90 o host stars will transit their

Dr. Matt Burleigh 3677: Life in the Universe

Transits

• • • Assuming – The whole planet passes in front of the star – And ignoring limb darkening as negligible Then the depth of the eclipse is simply the ratio of the planetary and stellar disk areas: – i.e. D f / f * = p R p 2 / p R * 2 = (R p / R * ) 2 We measure the change in magnitude planetary radius – Rem. D m = m transit – m * = 2.5 log (f • (smaller number means brighter) * / f transit ) D m, and obtain the stellar radius from the spectral type – Hence by converting to flux we can measure the Dr. Matt Burleigh 3677: Life in the Universe

Transits

Example: first known transiting planet HD209458b  D m = 0.017 mags – So (f * / f transit ) = 1.0158, i.e. D f=1.58% – From the spectral type (G0) R=1.15R

sun – So using D f / f * = (R p / R * ) 2 and setting f * =100% – Find R p =0.145R

sun – Since R sun =9.73R

J then – R p = 1.41R

J Dr. Matt Burleigh 3677: Life in the Universe

Transits

•

HD209458b more:

– From Doppler wobble method know M sin i = 0.62M

J – Transiting, hence assume i=90 o , so M=0.62M

J – Density = 0.29 g/cm 3 • c.f. Saturn 0.69 g/cm 3 – HD209458b is a gas giant! Dr. Matt Burleigh 3677: Life in the Universe

Transits

• • For an edge-on orbit, transit duration is given by:  D t = (PR * ) / ( p a) • Where P=period in days, a=semi-major axis of orbit Probability of transit (for random orbit) –

P transit

= R * / a – For Earth (P=1yr, a=1AU),

P transit

=0.5% – But for close, “hot” Jupiters,

P transit =

10% – Of course, relative probability of detecting Earths is lower since would have to observe for up to 1 year Dr. Matt Burleigh 3677: Life in the Universe

Transits

• • Advantages – Easy. Can be done with small, cheap telescopes • E.g. WASP, – Possible to detect low mass planets, including “Earths”, especially from space (Kepler mission, 2008) Disadvantages – Probability of seeing a transit is low • Need to observe many stars simultaneously – Easy to confuse with starspots, binary/triple systems – Needs radial velocity measurements for confirmation, masses Dr. Matt Burleigh 3677: Life in the Universe

Super WASP

• • • • • • • Wide Angle Search for Planets (by transit method) First telescope located in La Palma Operations started May ‘04 Data stored at Leicester Three new planets detected!

www.superwasp.org

www.wasp.le.ac.uk

Dr. Matt Burleigh 3677: Life in the Universe

Gravitational Microlensing

• • • • • • • A consequence of general relativity The grav. field of a relatively nearby star can bend the light of a more distant object as it passes in front of it, as seen from Earth The star doing the lensing brightens as a result We record this brightening, which can last for days If the lensed star has a planetary companion, the characteristic lensing light curve is modified Signals from an Earth-like planet would be strong (>5%) but brief (few hours) 4 planets found so far, including one at 5.5 Earth masses!

Dr. Matt Burleigh 3677: Life in the Universe

Direct detection

• • • Imaging = spectroscopy = physics: composition & structure No planet in orbit around another star has been directly imaged Why? – Stars like the Sun are billions of times brighter than planets – Planets and stars lie very close together on the sky • At 10pc Jupiter and the Sun are separated by 0.5” Dr. Matt Burleigh 3677: Life in the Universe

Direct detection

• • • • Problem 1: – Stars bright, planets faint Solution: – Block starlight with a coronagraph Problem 2: – Earth’s atmosphere distorts starlight, reduces resolution Solution: – Adaptive optics, Interferometry – difficult, expensive – Or look around very young and/or intrinsically faint stars (not Sun-like) Dr. Matt Burleigh 3677: Life in the Universe

First directly imaged planet?

• 2M1207 in TW Hya association • Discovered at ESO VLT in Chile • • • 25M jup Brown dwarf + 5M jup “planet” Distance ~55pc Very young cluster ~10M years • Physical separation ~55AU • A brown dwarf is a failed star – Can this really be called a planet? – Formation mechanism may be crucial!

Dr. Matt Burleigh 3677: Life in the Universe

Direct detection: White Dwarfs

• • • • • •

White dwarfs are the end state of stars like the Sun 1,000-10,000 times fainter than Sun-like stars

–

contrast problem reduced Outer planets should survive evolution of Sun to white dwarf stage, and migrate outwards

–

more easily resolved Over 100 WD within 20pc

–

At 10pc a separation of 100AU = 10” on sky I have a programme to search for planets around nearby WD with the Gemini 8m telescopes We call it “DODO” – Degenerate Objects around Degenerate Objects or Dead Objects etc

Dr. Matt Burleigh 3677: Life in the Universe

Direct Detection: White Dwarfs

Proper motions Two images taken one year apart •

The faint objects in this field could be massive planets in wide orbits around this nearby white dwarf

•

The white dwarf moves relatively quickly compared to background stars in the field (see movie)

•

If a faint object moves with the WD, then I would get excited

•

But in this case, there is nothing, but we could have detected something as small as ~5M Jup !

•

www.le.ac.uk/~mbu

Dr. Matt Burleigh 3677: Life in the Universe

What we know about

• • • • • • •

extra-solar planets

265 planets now found 25 multiple systems 33 transiting planets – can directly measure radii Unexpected population with periods of 3-4 days: “hot Jupiters” New population from transit surveys at ~2 days First planet with an orbit like Jupiter discovered (55 Cancri d) Is our solar system typical?

Dr. Matt Burleigh 3677: Life in the Universe

Extra-solar planet period distribution

• • • Notice the “pile up” at periods of 3-4 days / 0.04 0.05AU

The most distant planets discovered so far are at 5-6AU New discovery of transiting planets at ~2 days Dr. Matt Burleigh 3677: Life in the Universe

Extra-solar planet mass distribution

• • • Mass distribution peaks at 1-2 x mass of Jupiter Lowest mass planet so far: 5.5xM

Earth Super-Jupiters (>few M Jup ) are not common – Implications for planet formation theories?

– Or only exist in number at large separation?

– Or exist around massive stars?

Dr. Matt Burleigh 3677: Life in the Universe

Selection effects

• • • • • • Astronomical surveys tend to have built in biases These “selection effects” must be understood before we can interpret results The Doppler Wobble method is most sensitive to massive, close-in planets It is not yet sensitive to planets as small as Earth, even close-in As orbital period increases, the method becomes insensitive to planets less massive than Jupiter The length of time that the surveys have been active (since 1989) sets the upper orbital period limit – Only now are analogues of Jupiter in our own Solar System going to be found Dr. Matt Burleigh 3677: Life in the Universe

What we know about extra-solar planets: Mass versus semi-major axis

• • • • Blue – exoplanets Red – solar system Many of the known solar systems have ~Jupiter mass planets in small orbits, <0.1AU

– Selection effect of Doppler surveys But almost no super Jupiters are found in close orbits – Real, not a selection effect Dr. Matt Burleigh 3677: Life in the Universe

What we know about extra-solar planets

Eccentricity vs semi-major axis

: - large distribution of e (same as close binaries) - most extra-solar planets are on orbits much more eccentric than the giant planets in the solar system: bad news for survivability of terrestrial planets - planets close to the star are tidally circularized

extra-solar planets

- planets on circular orbits do exist far away from star

solar system planets

- the planets in our own system have small eccentricities ie STABLE Dr. Matt Burleigh 3677: Life in the Universe

•

Statistics of the Doppler Wobble surveys: Summary

Of 2000 stars surveyed – ~5% have gas giants between 0.02AU and 5AU • Trends suggest ~10% of stars have planets in orbits 5-7AU – 0.85% have hot Jupiters • Real effect – Hot Jupiters are not massive • Almost all have Msini~1M jup or less • No close-in, “super-Jupiters” – Mass distribution strongly peaks at 1M jup and falls as dN/dM~M -0.7

• But surveys currently biased towards hot Jupiters • Expect mass distribution to flatten somewhat as long periods, super-Jupiters are discovered Dr. Matt Burleigh 3677: Life in the Universe

What about the stars themselves?

• • • • Surveys began by targeting sun-like stars (spectral types F, G and K) Now extended to M dwarfs Incidence of planets is greatest for late F stars – F7-9V > GV > KV > MV – Few low mass M dwarfs known to have a planets despite ease of detectability Stars that host planets appear to be on average more metal-rich Dr. Matt Burleigh 3677: Life in the Universe

Metallicity

The abundance of elements heavier than He relative to the Sun

• • Overall, ~5% of solar-like stars have radial velocity –detected Jupiters But if we take metallicity into account: – >20% of stars with 3x the metal content of the Sun have planets – ~3% of stars with 1/3 rd of the Sun’s metallicity have planets Dr. Matt Burleigh 3677: Life in the Universe

Metallicity

• • • Does this result imply that planets more easily form in metal-rich environments?

– If so, then maybe planet hunters should be targeting metal-rich stars – Especially if we are looking for rocky planets This result also implies that chances of very old lifeforms (> few billion years) in the Universe are slim – With less heavy elements available terrestrial planets may be smaller and lower in mass than in our solar system – Is there a threshold metallicity for life to start (e.g. ½ solar)?

BUT Sigurdsson et al. (2003, Science, 301, 193) claim that a milli second pulsar in globular M4 has a Jupiter size companion – Claim based on timing anomalies – If true, then planets may have been forming 12 billion years ago in a very metal-poor environment (<0.1 x solar) – Alternatively, planet may have formed from debris of supernova explosion that created the pulsar – Or planet does not exist, timing anomalies have another cause Dr. Matt Burleigh 3677: Life in the Universe

Planet formation scenarios

• • • • • There are two main models which have been proposed to describe the formation of the extra-solar planets: Planets form from dust which agglomerates into cores which then accrete gas from a disc. A gravitational instability in a protostellar disc creates a number of giant planets.

Both models have trouble reproducing both the observed distribution of extra-solar planets and the solar-system.

Dr. Matt Burleigh 3677: Life in the Universe

Gas accretion onto cores

• • • • • • Planetary cores form through the agglomeration of dust into grains, pebbles, rocks and planetesimals within a gaseous disc At the smallest scale (<1 cm) cohesion occurs by non gravitational forces e.g. chemical processes.

On the largest scale (>1 km) gravitational attraction will dominate.

On intermediate scales the process is poorly understood. These planetesimals coalesce to form planetary cores and for the most massive cores these accrete gas to form the giant planets.

Planet formation occurs over 10 7 yrs.

Dr. Matt Burleigh 3677: Life in the Universe

Gravitational instability

• • • • • • • A gravitational instability requires a sudden change in disc properties on a timescale less than the dynamical timescale of the disc.

Planet formation occurs on a timescale of 1000 yrs.

A number of planets in eccentric orbits may be formed.

Sudden change in disc properties could be achieved by cooling or by a dynamical interaction.

Simulations show a large number of planets form from a single disc.

Only produces gaseous planets – rocky (terrestrial) planets are not formed.

Is not applicable to the solar system.

Dr. Matt Burleigh 3677: Life in the Universe

Where do the hot Jupiters come from?

• • • • No element will condense within ~0.1AU of a star since T>1000K Planets most likely form beyond the “ice-line”, the distance at which ice forms – More solids available for building planets – Distance depends on mass and conditions of proto planetary disk, but generally >1AU Hot Jupiters currently at ~0.03-0.04AU cannot have formed there Migration!

Dr. Matt Burleigh 3677: Life in the Universe

Planetary migration

• • •

Planets migrate inwards and stop when disk is finally cleared If migration time < disk lifetime

– Planets fall into star – Excess of planets at 0.03-0.04AU is evidence of a stopping mechanism in some cases – Nature of stopping mechanism unclear: tides? magnetic cavities? mass transfer?

Large planets will migrate more slowly

– Explanation for lack of super-Jupiters in close orbits Dr. Matt Burleigh 3677: Life in the Universe

Planetary migration & terrestrial planets

• • • Migrating giant planets will be detrimental to terrestrial planet survivability, if they both form at same time – Planets interior to a migrating giant planet will be disrupted and lost – Of course, these small planets may also migrate into star!

If terrestrial planets can only survive when migration doesn’t take place through their formation zone (few AU), – then 3%-20% of planet forming systems will possess them Alternatively, terrestrial planet formation may occur after dissipation of gas in proto-planetary disk (after 10 7 years) – Disruption by a migrating giant planet unlikely – Almost all planet-forming stars will have terrestrial planets Dr. Matt Burleigh 3677: Life in the Universe

The future: towards other Earths

• • • • • Pace of planet discoveries will increase in next few years Radial velocity surveys will reveal outer giant planets with long periods like our own Solar System Transit surveys will reveal planets smaller than Saturn in close orbits First direct images will be obtained But the greatest goal is the detection of other Earths Dr. Matt Burleigh 3677: Life in the Universe

Towards other Earths

Telescope

Corot Kepler GAIA SIM Darwin/TPF 50m ELT

Method

Transits

Date

2007 Transits Astrometry Interferometry 2012-15 (?) Interferometry 2015-20 (?) Imaging 2008 2012 2019 Dr. Matt Burleigh 3677: Life in the Universe

Towards Other Earths: Habitable Zones

Left: courtesy Prof. Keith Horne, St.Andrews

Right: courtesy Prof. Barry Jones, Open • • • Habitable zone defined as where liquid water exists Changes in extent and distance from star according to star’s spectral type (ie temperature) It is possible for rocky planets to exist in stable orbits of habitable zones of known hot Jupiter systems – If they were not previously cleared out by migration Dr. Matt Burleigh 3677: Life in the Universe

Towards Other Earths: Biomarkers

• • So we find a planet with the same mass as Earth, and in the habitable zone: – How can we tell it harbours life?

Search for biomarkers – Water – Ozone – Albedo Dr. Matt Burleigh 3677: Life in the Universe

Dr. Matt Burleigh 3677: Life in the Universe

Direct detection: proto-planetary disks

• • • • Dust disk around Fomalhaut Sub-mm image taken from James Clarke Maxwell telescope on Hawaii Disk has a hole in centre like a doughnut Part of disk appears to be perturbed Dr. Matt Burleigh 3677: Life in the Universe

Direct detection: proto-planetary disks

• •

Disk is 200 million years old

•

Like the early Solar System Is a planet perturbing the disk & forming the hole?

Dr. Matt Burleigh 3677: Life in the Universe

Direct detection: Epsilon Eridani

• • • • • Young, Sun-like star only 3pc away Dust disk is clumpy Clumps seen to rotate Requires presence of at least one Jovian planet 1.55M

Jup companion confirmed by HST measurements Dr. Matt Burleigh 3677: Life in the Universe