Infrared Signatures of Infrared Signatures of Planetary SystemsPlanetary Systems

Amaya Moro-MartinAmaya Moro-MartinDepartment of Astrophysical Sciences, Princeton UniversityDepartment of Astrophysical Sciences, Princeton University

Quick Tour to Star and Planet FormationQuick Tour to Star and Planet Formation

Stars form in clouds of dust and gas.Stars form in clouds of dust and gas.Local density increase occurs within these cloudsLocal density increase occurs within these clouds that portion of the cloud contracts in on itself that portion of the cloud contracts in on itself under its own gravitational pullunder its own gravitational pull a a protostarprotostar is formed is formed (no fusion yet).(no fusion yet).

By conservation of angular By conservation of angular momentum, what is left of momentum, what is left of the cloud rotates with the the cloud rotates with the protostar and begins to protostar and begins to flatten into a flatten into a circumstellar circumstellar disk.disk.

Some of this dust and gas Some of this dust and gas accretesaccretes onto the protostar onto the protostar adding to its mass.adding to its mass.

The disk is very dense. The disk is very dense. The grains are subject to The grains are subject to many forces and collide many forces and collide with each other often. with each other often. Some Some grainsgrains begin to begin to stick together. stick together.

Grains Grains grow grow until they form until they form planetesimalsplanetesimals (asteroid- (asteroid-size bodies); some of them grow even further into size bodies); some of them grow even further into small planets. The small planets. The terrestrial planetsterrestrial planets in our solar in our solar system are large accumulations of these bodies. system are large accumulations of these bodies.

Further from the central star, some of these large rocky Further from the central star, some of these large rocky cores cores accrete gasaccrete gas, forming , forming giant gas planetsgiant gas planets like Jupiter like Jupiter and Saturn.and Saturn.

The outcome resembles our solar system. Unfortunately, it is The outcome resembles our solar system. Unfortunately, it is the only planetary system we can observe in detail, so our the only planetary system we can observe in detail, so our view of planetary formation is biased. view of planetary formation is biased.

Why the IR and not the Why the IR and not the optical?optical?- In the optical, the light from - In the optical, the light from the star overpowers that of the the star overpowers that of the planet.planet.- The disk is completely dark, - The disk is completely dark, but it glows brightly in the but it glows brightly in the infrared.infrared.

Studying the evolution of disk Studying the evolution of disk properties (mass, radial structure ) properties (mass, radial structure ) and dust properties (size, and dust properties (size, composition).composition). Looking for warm molecular gas Looking for warm molecular gas (H(H22).).

For more mature systems, we can For more mature systems, we can trace evolution of dust disks trace evolution of dust disks generated through collisions of generated through collisions of planetesimals and planetesimals and infer location and infer location and mass of giant planets. mass of giant planets.

Define the timescales Define the timescales over which terrestrial over which terrestrial and gas giant planets and gas giant planets are built.are built.

Observations in the infrared can help us study other systemsObservations in the infrared can help us study other systems

Let’s see this in more detailLet’s see this in more detail

This dust is This dust is not primordialnot primordial but must be replenished by a but must be replenished by a reservoir of undetected reservoir of undetected planetesimalsplanetesimals producing dust by producing dust by mutual collisions. This is why we call them mutual collisions. This is why we call them debris disks.debris disks.

Debris disks are indirect evidence of planetary formation!!Debris disks are indirect evidence of planetary formation!!

With time, the remaining With time, the remaining dust in the disk dissipatesdust in the disk dissipates, it’s , it’s eithereither Blown away by the star due to Blown away by the star due to radiation pressureradiation pressure, or…, or… Drifts all the way into the star due to Drifts all the way into the star due to Poynting-Poynting-RobertsonRobertson dragdrag where it sublimates (timescale where it sublimates (timescale ~ 10~ 1055 -10 -1066 yrs) yrs)

Our Sun has Our Sun has a dust disk a dust disk too of 10too of 10-4-4 M M

However, many stars older than 10However, many stars older than 1077 yrs are yrs are still surrounded by dust disks (1-10M ) ?! still surrounded by dust disks (1-10M ) ?!

??

..and for a long time it was the only evidence we had…..and for a long time it was the only evidence we had…

Massive planets may scatter and Massive planets may scatter and eject eject dust particlesdust particles out of a planetary system out of a planetary system creating gaps.creating gaps.

Do debris disks harbor massive planets?Do debris disks harbor massive planets?As dust particles spiral inward (due to PR drag), they can As dust particles spiral inward (due to PR drag), they can get get trapped in Mean Motion Resonancestrapped in Mean Motion Resonances with the with the planets. I.e. massive planets shepherds the dust grains planets. I.e. massive planets shepherds the dust grains in the disks.in the disks.Without

planetswith Solar System planets

Uniform density diskUniform density disk

minimum at Neptune’s position (to avoid resonant planet)

Neptune

ring-like structure along Neptune’s orbit (trapping into Mean Motion Resonances)

clearing of dust from inner 10 AU (due to gravitational scattering by Jupiter and Saturn)

Massive planets sculpt the Massive planets sculpt the debris disks in which they are debris disks in which they are embeddedembedded

-Eri 850-Eri 850m (emitted m (emitted light; Greaves et al. light; Greaves et al. 98)98)

HR4796A 1.6 HR4796A 1.6 m m (scattered light; Schneider (scattered light; Schneider et al. 99)et al. 99)

H141569 1.1H141569 1.1m m (scattered light; (scattered light; Weinberger et al. Weinberger et al. 99)99)

Gaps and asymmetries observed in high-Gaps and asymmetries observed in high-resolution observations suggest giant planets resolution observations suggest giant planets may be present.may be present.

Structure is sensitive to Structure is sensitive to long period planetslong period planets complementary to radial complementary to radial

velocity and transit surveys.velocity and transit surveys. We can learn about the diversity of planetary We can learn about the diversity of planetary

systems from the study of debris disks structure! systems from the study of debris disks structure!

Needed to determine Needed to determine stability of orbits in stability of orbits in habitable zones (habitable zones (TPF)TPF)

Looking for planets in spatially unresolved Looking for planets in spatially unresolved disksdisks

Many disks are too far away to be spatially resolved Many disks are too far away to be spatially resolved in most cases we won’t be able to look for in most cases we won’t be able to look for planets planets by studying debris disk structure directly.by studying debris disk structure directly.But But the structure carved by the structure carved by the planets affects the the planets affects the shape of the Spectral shape of the Spectral Energy DistributionEnergy Distribution (SED) of (SED) of the diskthe disk

we can study the debris we can study the debris disk structure indirectly.disk structure indirectly.

Let’s see some modeled SEDs of debris disks with Let’s see some modeled SEDs of debris disks with embedded planets in different configurations.embedded planets in different configurations.

Infraredexcess

Log

[F(m

Jy)]

No planet

star 1AU5AU 30AU

Log[m)]

50AU

Carbonaceous grains

Fe-rich silicate grains

Fe-poor silicate grains

Planetesimals (Kuiper Belt)

Log

[F(m

Jy)]

1 MJup at 5 AU

star 1AU5AU 30AU

Log[m)]

50AU

Carbonaceous grains

Fe-rich silicate grains

Fe-poor silicate grains

Planetesimals (Kuiper Belt)

Log

[F(m

Jy)]

3 MJup at 1 AU

star 1AU5AU 30AU

Log[m)]

50AU

Carbonaceous grains

Fe-rich silicate grains

Fe-poor silicate grains

Planetesimals (Kuiper Belt)

Log

[F(m

Jy)]

3 MJup at 5 AU

star 1AU5AU 30AU

Log[m)]

50AU

Carbonaceous grains

Fe-rich silicate grains

Fe-poor silicate grains

Planetesimals (Kuiper Belt)

Log

[F(m

Jy)]

3 MJup at 30AU

star 1AU5AU 30AU

Log[m)]

50AU

Carbonaceous grains

Fe-rich silicate grains

Fe-poor silicate grains

Planetesimals (Kuiper Belt)

The SED of a dust disk with embedded planets is The SED of a dust disk with embedded planets is fundamentally different from that of the disk without fundamentally different from that of the disk without planets.planets. Significant decrease of the near/mid-IR flux Significant decrease of the near/mid-IR flux due to the clearing of dust inside the planet’s due to the clearing of dust inside the planet’s orbit.orbit.

It may be It may be possiblepossible to diagnose the to diagnose the locationlocation of the of the planet and the absence/presence of planetsplanet and the absence/presence of planets

What could we learn from the Spectral What could we learn from the Spectral Energy Distributions?Energy Distributions?

Spitzer Space TelescopeSpitzer Space Telescope observations of debris observations of debris

disksdisks

SpitzerSpitzer has identify the first stars with well-confirmed planetary has identify the first stars with well-confirmed planetary systems and well-confirmed IR excess!!systems and well-confirmed IR excess!!

Study of 26 FGK stars with confirmed radial velocity planets Study of 26 FGK stars with confirmed radial velocity planets (average age ~ 1 Gyr): (average age ~ 1 Gyr):

6/26 show 70 6/26 show 70 mm excess excess (average age ~ 4 Gyr).(average age ~ 4 Gyr).none with 24 none with 24 m excessm excess: upper limit of warm dust L: upper limit of warm dust Ldustdust/L/Lstarstar

~5x10 ~5x10-5-5 (compared to L(compared to Ldustdust/L/Lsun sun ~10~10-7 -7 for for the solar system’s asteroid belt the solar system’s asteroid belt dust). dust).

Similar to Kuiper Belt dust disk: T<100K; >10AU; Similar to Kuiper Belt dust disk: T<100K; >10AU; 100 x surface emitting area of the solar system’s 100 x surface emitting area of the solar system’s KB dust.KB dust.

Potential correlation of planets with IR excess: Potential correlation of planets with IR excess: 4/5 of the 4/5 of the largest 70 largest 70 m detections are for stars with RV planets, even though m detections are for stars with RV planets, even though the planet bearing stars make up <1/3 of the sample.the planet bearing stars make up <1/3 of the sample.

Debris Disks and planets co-exist!Debris Disks and planets co-exist! (Beichman et al. 2005) (Beichman et al. 2005)

Cold KB-like disks appear to be more Cold KB-like disks appear to be more common than AB-like diskscommon than AB-like disks (Hines et al. 2005) (Hines et al. 2005)

Only 1 out of 33 stars (with ages between 10 Myr and 2 Gyr) Only 1 out of 33 stars (with ages between 10 Myr and 2 Gyr) have warm excesses: have warm excesses:

- Are these excesses - Are these excesses short livedshort lived events connected with events connected with the the formation of terrestrial planets? or... formation of terrestrial planets? or...

- Is dust production in terrestrial planet-building zones - Is dust production in terrestrial planet-building zones rare?rare?

HD12039HD12039 (30 Myr). (30 Myr).Strong emission at 24 Strong emission at 24 m:m: AB-like disk in terrestrial planet region AB-like disk in terrestrial planet region (T=100-300K). L(T=100-300K). LIRIR /L /Lstarstar ~ 10 ~ 10-4-4

Not detected at 70 Not detected at 70 m:m: rule out KB-like dust between 10-30AU.rule out KB-like dust between 10-30AU.No prominent spectral features: grain size > 3-10 No prominent spectral features: grain size > 3-10 mm located located between 4-6AU.between 4-6AU.Lifetime (due to PR) < 2 Myr (<stellar age): dust is being Lifetime (due to PR) < 2 Myr (<stellar age): dust is being regenerated. Either there is a huge reservoir of material or the regenerated. Either there is a huge reservoir of material or the dust is due to a recent collisional event.dust is due to a recent collisional event.

Individual collisional events can dominate the Individual collisional events can dominate the properties of debris disks over Myr timescales (A star properties of debris disks over Myr timescales (A star survey)survey)

[For Vega: a dust production rate of 10[For Vega: a dust production rate of 101515g/s over the age of g/s over the age of Vega (350Myr) would produce ~ 6MVega (350Myr) would produce ~ 6MJup Jup of dust (very of dust (very unlikely!)].unlikely!)].

Overall decay in the Overall decay in the maximum 24 maximum 24 m excess with m excess with age.age.

50% of young stars have no 50% of young stars have no 24 24 m excess m excess (in some cases (in some cases there is very little material there is very little material between 10 and 60 AU after proto-between 10 and 60 AU after proto-planetary disk is cleared).planetary disk is cleared).

Stars of a similar age show Stars of a similar age show substantial differences in the substantial differences in the amount of dust!amount of dust!

(Rieke et al. 2005, Su et al. (Rieke et al. 2005, Su et al. 2005)2005)

Inner gaps appear to be Inner gaps appear to be common in cold KB-like diskscommon in cold KB-like disks (Kim et al. 2005, Meyer et al. 2004)(Kim et al. 2005, Meyer et al. 2004)

70 70 mm excesses: T excesses: Tmaxmax < 100K, < 100K, RRinin>10AU >10AU

No 24 No 24 mm excesses: Upper limit of excesses: Upper limit of warm dust inside Rwarm dust inside Rinin ~ 10 ~ 10-6-6-10-10-6.5-6.5 M MEarth Earth 2-3 orders of magnitude below the 2-3 orders of magnitude below the lower limits for the masses in the cold lower limits for the masses in the cold disk. disk.

Large depletion inside RLarge depletion inside Rinin

Lifetimes (due to PR) ~ 10Lifetimes (due to PR) ~ 106 6 yryr- Replenishment of dust- Replenishment of dust- PR would erase the density - PR would erase the density contrast inside and outside Rcontrast inside and outside Rinin

What is stopping the particles from What is stopping the particles from drifting all the way toward the star?drifting all the way toward the star?

(Kim et al. 2005)(Kim et al. 2005)

Sublimation of icy grains? No, T<100K.Sublimation of icy grains? No, T<100K.

Blowout by radiation pressure? No, dust grains are Blowout by radiation pressure? No, dust grains are large enough to be on bound orbits.large enough to be on bound orbits.

An interesting possibility: An interesting possibility: scattering by a massive scattering by a massive planet. planet.

If the planet is in a circular orbit the If the planet is in a circular orbit the models predict the planet to be located (0.8-models predict the planet to be located (0.8-1.25)xR1.25)xRinin, with a mass significantly larger that , with a mass significantly larger that Neptune and probably larger than Jupiter.Neptune and probably larger than Jupiter.

Inner gap Inner gap radiusradius

(Kim et al. (Kim et al. 2005)2005)

SummarySummaryDebris Disks are evidence of planetary formation (because Debris Disks are evidence of planetary formation (because planetesimals are needed to generate the dust).planetesimals are needed to generate the dust).Massive planets create structure in debris disks and high Massive planets create structure in debris disks and high resolution observations show that structure is indeed resolution observations show that structure is indeed present.present.Structure is sensitive to Structure is sensitive to long period planets, long period planets, complementing radial velocity and transit surveys. complementing radial velocity and transit surveys. Debris disk help us learn about diversity of planetary Debris disk help us learn about diversity of planetary systems.systems.The clearing of dust inside the planet’s orbit has a clear The clearing of dust inside the planet’s orbit has a clear signature in the disk SED signature in the disk SED SEDs are sensitive to the SEDs are sensitive to the presence and location of massive planets.presence and location of massive planets. Spitzer Space TelescopeSpitzer Space Telescope observations of debris disks: observations of debris disks: Debris disks and planets co-exist.Debris disks and planets co-exist. Cold KB-like disks are more common than AB-like disks.Cold KB-like disks are more common than AB-like disks. Individual collisional events may dominate disk Individual collisional events may dominate disk properties.properties. Inner gaps appear to be common in cold KB-like disksInner gaps appear to be common in cold KB-like disks May indicate that massive long-period planets are also May indicate that massive long-period planets are also common!common!

Is the Is the “late bombardment”“late bombardment” epoch in the early Solar epoch in the early Solar System common among other stars? Is its intensity System common among other stars? Is its intensity below or above average? below or above average?

Consequences for the survival of Life in theConsequences for the survival of Life in the terrestrial planets. terrestrial planets.

Astrobiology linkAstrobiology linkBy studying these disks we can: By studying these disks we can: Study frequency and timescale of terrestrial planet Study frequency and timescale of terrestrial planet formation, constraining theories of planetary formation.formation, constraining theories of planetary formation. Study the diversity of planetary systems, allowing Study the diversity of planetary systems, allowing us to put our solar system into context by comparing it us to put our solar system into context by comparing it to other planetary systems. to other planetary systems.

Is our solar system (in it’s evolution and Is our solar system (in it’s evolution and planetary configuration) common or rare?planetary configuration) common or rare?