PHYS178 2008 week 11 part-1

Extrasolar planetsLecture 2: Planetary formation theory

and detection techniquesA/Prof. Quentin A Parker

PHYS178 - other worlds: planets and planetary systems 1

Formation of a Star and proto-planetary disk

(a)Dense cores form within a molecular cloud. (b) A protostar with a surrounding disk of material forms at the centre, accumulating additional material from the molecular cloud through gravitational attraction. (c) A stellar wind breaks out, confined by the disk to flow along the stellar poles. (d) Eventually this wind sweeps away the cloud and halts the accumulation of additional material, and a newly formed star, surrounded by a disk, becomes visible. The diameter of a typical accreting envelope is about 5000 astronomical units. The typical diameter of the disk is about 100 AU.

Disks around protostars

These Hubble Space Telescope infrared images show disks around young stars in the constellation of Taurus, in a region about 450 LY away.

In some cases we can see the central star (or stars—some are binaries). In other cases, the dark horizontal bands indicate regions where the dust disk is so thick that even infrared radiation from the star embedded within it cannot make its way through.

The bright glowing regions are starlight reflected from the upper and lower surfaces of the disk, which are less dense that the central regions.

Fig 20-12, p.450

•Tracks are plotted on the H–R diagram to show how stars of different masses change during the early parts of their lives.

•The numbers next to each dark point on a track are the rough number of years it takes an embryo star to reach that stage.

•You can see that the more mass a star has, the shorter the time it takes to go through each stage. •Stars that lie above the dashed line would typically still be surrounded by infalling material and would be hidden by it.

Disks around protostars

These HST images show 4 disks around young stars in the Orion Nebula.

The dark, dusty disks are seen silhouetted against the bright backdrop of the glowing gas in the nebula.

The size of each image is about 30 times the diameter of our planetary system;this means the disks we see here range in size from two to eight times the orbit of Pluto.

The red glow at the center of each disk is a young star, no more than a million years old.

(credit: M. McCaughrean, C. R. O’Dell, and NASA)


• The currently favoured scenario for planet formation is that of core accretion• Initially planetary cores form from condensed material in the protoplanetary disc around a star• In an inner hotter zone only grains of dust and small particulates aggregate together• Planet formation is further supported by the presence of icy snowballs in a cooler zone outside the so-called“ice boundary”• Planets forming there are likely to grow to gas giants by accreting hydrogen and helium• They can then migrate inwards• A decent fraction end up in close orbits • These constitute the hot inner planets.

This near infrared HST image shows a narrow ring of dust around the very young star HR 4796A, ~220 lyr away in the constellation of Centaurus. The ring is very narrow, spanning the same distance as that which separates Mars from Uranus Though the ring is much further from its star, lying at what would be about twice the distance of Pluto from our Sun. The image was taken with a coronagraph, a device that covers the bright star which allows faint structures to be seen.(B. Smith, U. of Hawaii; G. Schneider, U. of Arizona; and NASA)

Dust Ring Around a Young Star

Planets do not produce (much of )their own

light

They are very far away

The stars they orbit are too bright


(at least not at the moment)

We have to rely on indirect methods

The Doppler shift Astrometry Planet transits Gravitational microlensing Direct imaging



Planets do not orbit stars, they orbit each

other around the common centre of

mass

This causes the star to

“wobble”

In 1842, the Austrian physicist Christian Johann Doppler noted that the wavelength of light, sound, or any other kind of propagating energy measured by a moving observer will be shifted by a factor of: v/c where v is the velocity at which the observer is approaching or receding from the source and c is the speed at which the wave propagates

This effect occurs for any kind of radiation not just electromagnetic radiation

We may all be familiar with the effect with sound – a mechanical wave transmitted through air

We note the doppler effect with sound by a change in pitch of the sound

17

Consider truck approaching with constant velocity clanging bell once a sec.

When bell clangs first the sound reaches our ears and 1 sec later the truck has moved forward and the sound from the bell has a shorter distance to travel

Note the circles of expanding sound from the position of each bell causing a compression of sound ahead of the truck and an expansion behind due to the truck’s movement

18

light waves emanate from origin If source is moving forward relative to observer (a)

then the wavefront is compressed – frequency is increased and wavelength decreased (blue shift)

If source receding from observer (b) then wavefront is stretched out, i.e. the frequency is decreased and wavelength is increased (red shift)

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v(a) (b)

Measure this wobble using spectroscopy


Convert Doppler shift into velocity

Derive Period, Eccentricity & minimum Mass

Velocity measured in metres per second

Required precision is ~3 m/s or1 part in 100,000,000!


Conception Reality ~ 40 years

Use very stable spectrographs, either

Very temperature/pressure stable; or

With very precise reference

Several long-term programmes


1952

Measure the absolute position of an object over time

Look for a regular wobble as the star drifts through space

Very hard to do - need to account for all other effects


Need accuracy to 1 part in 10,000,000!

Similar to a solar eclipse

A star dims when a planet passes in front

Brightness change depends on the

planet’s size

Small planet small change

Large planet large change


Only a small fraction of planets transit

Almost any telescope can be used to find

them

An amateur astronomer discovered a planet

around HD149026 with a 14” Celestron

Space telescopes have a huge advantage



From Einstein’s Theory of General

Relativity

The gravity of massive objects

can act as a “lens”


Stars can do this on a smaller scale

A background star will brighten when a star passes in front of it

More sensitive than other techniques to small-mass earth-like planets

Most sensitive to planets that have orbits of just several AU’s (such as for Mars or Jupiter/Saturn)

The most common stars will be the most likely candidates for lensing

Capable of detecting multiple planets in a single light curve

Can be used to study the statistical abundance of extra solar planets in our own Galaxy with properties akin to those in the Solar System.


Millions of stars must be monitored to find the few that are microlensing at any given time

Planetary deviations in a light curve are short-lived and could be easily be missed

Quite high probability that any planet will not be detected in a lensed system, even if present

Deviations in microlensing light curves due to planets will not repeat (as they are due to a chance alignment that will not recur

Planetary parameters (such as mass, orbit size, etc) depend on the properties of the host star, which are typically unknown

The microlensing technique requires intensive use of telescope time, and is unsuitable for continued detailed study of individual extra solar planets


Table 20-2, p.457

Astronomers would, however, prefer to obtain a direct image of an exoplanet, allowing them to better characterize the object's physical nature. This is an exceedingly difficult task, as the planet is generally hidden in the "glare" of its host star.

To partly overcome this problem, astronomers study very young objects. Indeed, sub-stellar objects are much hotter and brighter when young and therefore can be more easily detected than older objects of similar mass.

Based on this approach, it might well be that last year's detection of a feeble speck of light next to the young brown dwarf 2M1207 by an international team of astronomers using the ESO Very Large Telescope (ESO PR 23/04) is the long-sought bona-fide image of an exoplanet. A recent report based on data from the Hubble Space Telescope seems to confirm this result. The even more recent observations made with the Spitzer Space Telescope of the warm infrared glows of two previously detected "hot Jupiter" planets is another interesting result in this context. This wealth of new results, obtained in the time span of a few months, illustrates perfectly the dynamic of this field of research.

On several occasions during the past years, astronomical images revealed faint objects, seen near much brighter stars. Some of these have been thought to be those of orbiting exoplanets, but after further study, none of them could stand up to the real test. Some turned out to be faint stellar companions, others were entirely unrelated background stars. This one may well be different.

In April of this year, the team of European and American astronomers detected a faint and very red point of light very near (at 0.8 arcsec angular distance) a brown-dwarf object, designated 2MASSWJ1207334-393254. Also known as "2M1207", this is a "failed star", i.e. a body too small for major nuclear fusion processes to have ignited in its interior and now producing energy by contraction. It is a member of the TW Hydrae stellar association located at a distance of about 230 light-years. The discovery was made with the adaptive-optics supported NACO facility [3] at the 8.2-m VLT Yepun telescope at the ESO Paranal Observatory (Chile).

The feeble object is more than 100 times fainter than 2M1207 and its near-infrared spectrum was obtained with great efforts in June 2004 by NACO, at the technical limit of the powerful facility. This spectrum shows the signatures of water molecules and confirms that the object must be comparatively small and light.

None of the available observations contradict that it may be an exoplanet in orbit around 2M1207. Taking into account the infrared colours and the spectral data, evolutionary model calculations point to a 5 jupiter-mass planet in orbit around 2M1207. Still, they do not yet allow a clear-cut decision about the real nature of this intriguing object. Thus, the astronomers refer to it as a "Giant Planet Candidate Companion (GPCC)" [4].

Observations will now be made to ascertain whether the motion in the sky of GPCC is compatible with that of a planet orbiting 2M1207. This should become evident within 1-2 years at the most.

ESO PR Photo 26a/04 is a composite image of the brown dwarf object 2M1207 (centre) and the fainter object seen near it, at an angular distance of 778 milliarcsec. Designated "Giant Planet Candidate Companion" by the discoverers, it may represent the first image of an exoplanet. Further observations, in particular of its motion in the sky relative to 2M1207 are needed to ascertain its true nature. The photo is based on three near-infrared exposures (in the H, K and L' wavebands) with the NACO adaptive-optics facility at the 8.2-m VLT Yepun telescope at the ESO Paranal Observatory.

ESO PR Photo 26b/04 shows near-infrared H-band spectra of the brown dwarf object 2M1207 and the fainter "GPCC" object seen near it, obtained with the NACO facility at the 8.2-m VLT Yepun telescope. In the upper part, the spectrum of 2M1207 (fully drawn blue curve) is compared with that of another substellar object (T513; dashed line); in the lower, the (somewhat noisy) spectrum of GPCC (fully drawn red curve) is compared with two substellar objects of different types (2M0301 and SDSS0539). The spectrum of GPCC is clearly very similar to these, confirming the substellar nature of this body. The broad dips at the left and the right are clear signatures of water in the (atmospheres of the) objects.

"If the candidate companion of 2M1207 is really a planet, this would be the first time that a gravitationally bound exoplanet has been imaged around a star or a brown dwarf" says Benjamin Zuckerman of UCLA, a member of the team and also of NASA's Astrobiology Institute.

Using high-angular-resolution spectroscopy with the NACO facility, the team has confirmed the substellar status of this object - now referred to as the "Giant Planet Candidate Companion (GPCC)" - by identifying broad water-band absorptions in its atmosphere, cf. PR Photo 26b/04.

The spectrum of a young and hot planet - as the GPCC may well be - will have strong similarities with an older and more massive object such as a brown dwarf. However, when it cools down after a few tens of millions of years, such an object will show the spectral signatures of a giant gaseous planet like those in our own solar system.

Although the spectrum of GPCC is quite "noisy" because of its faintness, the team was able to assign to it a spectral characterization that excludes a possible contamination by extra-galactic objects or late-type cool stars with abnormal infrared excess, located beyond the brown dwarf.

After a very careful study of all options, the team found that, although this is statistically very improbable, the possibility that this object could be an older and more massive, foreground or background, cool brown dwarf cannot be completely excluded. The related detailed analysis is available in the resulting research paper that has been accepted for publication in the European journal Astronomy & Astrophysics (see below).

Implications

The brown dwarf 2M1207 has approximately 25 times the mass of Jupiter and is thus about 42 times lighter than the Sun. As a member of the TW Hydrae Association, it is about eight million years old.

Because our solar system is 4,600 million years old, there is no way to directly measure how the Earth and other planets formed during the first tens of millions of years following the formation of the Sun. But, if astronomers can study the vicinity of young stars which are now only tens of millions of years old, then by witnessing a variety of planetary systems that are now forming, they will be able to understand much more accurately our own distant origins.

Anne-Marie Lagrange, a member of the team from the Grenoble Observatory (France), looks towards the future: "Our discovery represents a first step towards opening a whole new field in astrophysics: the imaging and spectroscopic study of planetary systems. Such studies will enable astronomers to characterize the physical structure and chemical composition of giant and, eventually, terrestrial-like planets."

ESO PR Photo 10a/05 shows the VLT NACO image, taken in the Ks-band, of GQ Lupi. The feeble point of light to the right of the star is the newly found cold companion. It is 250 times fainter than the star itself and it located 0.73 arcsecond west. At the distance of GQ Lupi, this corresponds to a distance of roughly 100 astronomical units. North is up and East is to the left.

ESO PR Photo 10c/05 shows the NACO spectrum of the companion of GQ Lupi (thick line, bottom) in the near-infrared (around the Ks-band at 2.2 microns). For comparison, the spectrum of a young M8 brown dwarf (top, in red) and of a L2 brown dwarf (second line, in brown) are shown. Also presented is the spectrum calculated using theoretical models for an object having a temperature of 2,000 degrees. This theoretical spectrum compares well with the observed one.

PHYS178 2008 week 11 part-1

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