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Figure 3.

physics) and derive their masses and temperatures from

HR 8799bcd synthetic spectra. The open circles are the observed photometric data points (from Keck) plotted with 3-sigma error bars. The filled symbols are the synthetic spectra magnitudes. The synthetic partially cloudy spectrum of each planet predicts the presence of water absorption bandheads in between the J-, H-, K-, and L’-band bandpasses, as well as a mild 1.6-micron methane absorption bandhead for planet “b.”

exoplanet cooling tracks (see Figure 2, right). All three objects fall below the dividing line between planets and brown dwarfs (which exhibit short-lived deuterium burning) for the estimated age range of the system (30160 million years). When compared with the observed photometry, the synthetic spectra derived from the two extreme atmosphere models (fully dusty and dust free) were both unsatisfactory, showing that all three planets are in the transition region between these two extremes. Travis Barman, a member of the team, then constructed intermediate dust cloud atmosphere models by modifying the Phoenix atmospheric code. The result is a fairly good match between the synthetic spectrum and the observed photometry and luminosity of each of the three planets. In addition, the latest HR 8799b HST F160W detection is consistent with the Near-infrared Camera and Multi-Object Spectrometer

presence of water absorption in its atmosphere and

(NICMOS) in 1998. The three planets are ~ 50,000 times fainter than HR 8799 at 2.2 microns (K-band) and

in agreement with the partially cloudy atmospheric

are located at projected separations of 0.63, 0.95, and

all three objects (HR 8799bcd) are: 7-10 Jupiter masses,

1.73 arcseconds (24, 38, and 68 AU, respectively). The

a radius of ~ 1.2 times that of Jupiter, and a temperature of ~ 1000K, thus all three objects are young and warm,

measurements from all available epochs confirm that all three objects are co-moving with the star and display

models (see Figure 3). The final physical parameters of

partially cloudy (dust clouds) gas giant planets.

counterclockwise orbital motions that are consistent with Keplerian orbits.

Did the planets around HR 8799 form in the same way as the planets did in our solar system? The worlds of

The characterization of the three planets’ atmospheres

our solar system all orbit around the Sun in the same

is difficult due to their low luminosity relative to the

direction, in orbital planes close to the solar equator.

primary. Near-infrared spectroscopy with an integral

This suggests that the planets formed in a disk around

field spectrograph would require a large amount

the Sun. They may have formed through the process of

of telescope time. Instead, we have decided to first

core accretion, where small dust particles agglomerate

concentrate on acquiring accurate photometry of all

to form bigger rocks until planet cores are formed,

three objects at 1.2 microns (J-band), 1.6 microns (H-band

which then triggers a runaway accretion that attracts

and methane on/off bands), 2.2 microns (K-band), and 4

a large fraction of the remaining gas and dust to form

microns (L’-band)—a wavelength regime where most of

gas giant planets.

the planet’s light is emitted. The colors of these objects are significantly different from those of field brown

The planets around HR 8799 show a lot of resemblance

dwarfs (objects between 13.6 and 75 Jupiter masses,

to this formation scenario. All three orbit HR 8799

see Figure 2, left), and closer to those of 2M1207b (a

in the same counterclockwise direction. The measured

~ 7 Jupiter-mass planetary companion in orbit around a brown dwarf ) and two ~ 11 Jupiter-mass Pleiades

orbital motions are nearly perpendicular to the lines

candidate members. The much redder colors of these

mostly circular orbits viewed roughly face-on. All three

objects are due to their lower surface gravities (thus,

planets would thus be orbiting in similar orbital planes.

lower mass) and dust cloud physics.

The star rotation is also very slow compared to other

connecting the planets to the star, consistent with

A-type stars, consistent with a pole-on view. Thus, the


Since the acquired multi-band photometry is sensitive

planets would be orbiting in similar planes close to the

to most of the emitted light from these objects, it was

star’s equator. For these reasons, we conclude that the

possible to estimate their total luminosities (a value

three planets almost certainly formed in a disk around

that is independent of the details of their atmospheric

HR 8799.


Issue 38 - June 2009