The Light of Distant Worlds

by Paul Gilster on March 23, 2005

As discussed in yesterday’s entry, being able to work with actual light from distant planets is a major breakthrough. It opens the possibility of studying characteristics like temperature and atmospheric composition, further fusing astronomy with the nascent science of astrobiology. And with the Spitzer Space Telescope’s proven ability to make such observations, we can expect a whirlwind of exoplanetary data ahead.

A few further details from yesterday’s announcements:

A study of the work on HD 209458b, a ‘hot Jupiter’ that orbits its parent star in 3.5 days, ran in today’s online edition of Nature. The paper is Deming, D., Seager, S. et al., “Infrared radiation from an extrasolar planet.” Dr. Sara Seager of the Carnegie Institution, a co-author of the study, provided more about HD 209458b:

“This planet was discovered indirectly in 1999 and was later found to transit its star–the star dims as the planet moves in front of it during the course of the planet’s orbit. With Spitzer, we first measured the combined light of the planet and star just before the planet went out of sight. Then when the planet was out of view, we measured how much energy the star emitted on its own. The difference between those readings told us how much the planet emitted.”

The result: HD 209458b was found to be a scorching 1,574 F (1130 K).’s news site also offers the article “Light from alien planets,” by Mark Peplow, a useful overview.

Diagram of Spitzer planet findOther facts: HD 209458b lies 153 light years from Earth in the constellation Pegasus. TrES-1 is 489 light years away in the constellation Lyra. And while yesterday’s entry said that both HD 209458b and TrES-1 orbit stars very much like our sun, this is true only of the former; TrES-1’s star is smaller and cooler.

Image: Frame from a video simulation shows in a simplified schematic how the brightness of a star/planet system varies as the planet is eclipsed by the star. The false colors represent infrared images. Credit: NASA/JPL/Caltech-R. Hurt.

The paper on TrES-1 by David Charbonneau et al. is in press at The Astrophysical Journal, with publication slated for the June 20 issue. A preprint of the paper, “Detection of Thermal Emission from an Extrasolar Planet,” is available at the ArXiv site.

Also intriguing on the exoplanet front is planet-hunter Geoff Marcy’s intention to issue a catalog that will cover all exoplanets found to date. Marcy’s own team has thus far found almost 100 planets, and he is now turning to The Planetary Society in search of donations for the catalog.

Why is a catalog necessary? Here’s what The Planetary Society has to say in a recent release:

…most of the data is not available to the scientific community, or to the public at large. Of course, preliminary information about each world was published as it was discovered. But scientists have continued to study them, adding mountains of data to those initial observations.

So, while a massive reservoir of fantastic information about these new worlds now exists… almost no one can tap into it because the new data has never been properly processed and released. There could be critical discoveries buried in those mounds of data that have never seen the light of day.

Centauri Dreams agrees that such an updatable resource would be of scientific importance as well as providing broad educational options for students and teachers.

ljk August 24, 2007 at 13:17

Subaru Measures the Spin-Orbit Alignment of a Faint Transiting
Extrasolar Planetary System


Principal Investigator
Dr. Norio Narita, University of Tokyo, +81-3-5841-4177 (JAPAN, GMT+9 hours)
Public Information and Outreach
Dr. Tetsuharu Fuse, Subaru Telescope, +1-808-934-5922 (USA, GMT-10 hours)

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Subaru Measures the Spin-Orbit Alignment of a Faint Transiting
Extrasolar Planetary System

A joint Japanese/U.S. collaboration has used the Subaru Telescope’s
High Dispersion Spectrograph (HDS) to observe the transiting
extrasolar planetary system TrES-1. The observation, which allowed
the team to measure the angle between the parent star’s spin axis
and the planet’s orbital axis, is only the third time such a
spin-orbit alignment has been measured. In addition, TrES-1 is the
faintest target ever used for such a determination.

TrES-1 has a visual magnitude of about 12, which is only about 2% as
bright as the previous targets) used for such a measurement. The
team’s work has conclusively demonstrated that spin-orbit alignments
can be measured by studying the radial velocity anomaly introduced
during a transit (the so-called Rossiter-McLaughlin effect).

According to team leader Norio Narita, a graduate student at the
University of Tokyo (Japan Society for Promotion of Science Fellow,
DC2), this is important because most of the newly discovered
transiting planets from ongoing transit surveys have relatively faint
host stars. “By combining future observations of the
Rossiter-McLaughlin effect in other transiting systems,” said Narita.
“We will be able to determine the distribution of the spin-orbit
alignment angles for exoplanetary systems. Moreover, further
observations would have the potential to discover large spin-orbit
misalignments, if any, which would inspire numerous theoretical

More than 200 extrasolar planets have been discovered so far. The
discovery and characterization process has revealed a diversity of
planetary systems, and a variety of theoretical models have been
proposed to explain the complex process of planet formation. The
alignment of the stellar spin axis and the planetary orbital axis
(Figure 1) is a promising diagnostic for using observational data to
characterize planet formation mechanisms. For example, models
considering giant planet scattering (outward through the
protoplanetary disk) often predict tilts different from the original
orbital axis. It follows that such planets would usually have
significant misalignments. However, planets which form and migrate
inward through their birth disks would generally have negligible

In transiting extrasolar planetary systems (Note 1) (those where
planets cross in front of and behind their stars from our point of
view), one can measure the spin-orbit alignments by exploiting the
Rossiter-McLaughlin effect (Figure 2). Prior to the Subaru team’s
work, measurements of two bright transiting systems were conducted by
an international team using the Keck Telescope, led by Prof. Joshua
Winn (MIT), and a co-investigator on the Subaru team. The TrEs-1
observing collaboration, which also used the MAGNUM 2-meter telescope
at Haleakala, Hawai’i, (Figure 3) in addition to the Subaru High
Dispersion Spectrograph, succeeded in detecting the
Rossiter-McLaughlin effect (Figure 4) for the first time for that
target and constrained the alignment angle to 30 degrees with an
error of plus or minus 21 degrees and clearly indicating the prograde
orbital motion of TrES-1b.

This result will be published in the August 25, 2007 issue of
Publications of Astronomical Society of Japan.


Measurement of the Rossiter–McLaughlin Effect
in the Transiting Exoplanetary System TrES-1
Narita, N., Enya, K., Sato, B., Ohta, Y., Winn, J. N., Suto, Y., Taruya, A.,
Turner, E. L., Aoki, W., Tamura, M., Yamada, T., Yoshii, Y. 2007,
Publ. Astron. Soc. Japan, vol 59, No. 4, 763-770

Extrasolar planetary systems in which a planet’s orbit passes in front of
its host star (namely, causing an eclipse) are called transiting extrasolar
planetary systems.

TrES-1 is a main sequence K0 star and its planet TrES-1b was discovered by
the transit survey in 2004. TrES-1b is a gaseous giant orbiting the host star
with a period of about three days (one of the so called “hot Jupiter” class of
extrasolar planets).

An illustration of the concept of the spin-orbit alignment (indicated by
lambda) in an exoplanetary system.

The Rossiter-McLaughlin effect is defined as the radial velocity anomaly
during a transit from the known Keplerian orbit caused by the partial
occultation of the rotating stellar disk. For example, if a planet occults
part of the blue-shifted (approaching) half of the stellar disk, then the
radial velocity of the star will appear to be slightly red-shifted, and
vice-versa. The radial velocity anomaly depends on the trajectory of the
planet across the disk of the host star, and in particular on the spin-orbit
alignment of the system. Thus by monitoring the Rossiter-McLaughlin effect
one can measure the spin-orbit alignment.

A photometric light curve of TrES-1 from the MAGNUM observation (top),
and radial velocities obtained with the Subaru/HDS (bottom). The light curve
shows that the observations were conducted around a transit of TrES-1b.

Orbital plots of TrES-1 radial velocities and the best-fitting models
including the Kepler motion and the Rossiter-McLaughlin effect.
Left panel: A radial velocity plot for the whole orbital phase.
Right panel: A close-up of the radial velocity plot around the transit phase.
The waveform around the central transit time (phase = 0) is caused by the
Rossiter-McLaughlin effect.
Bottom panels: Residuals from the best-fit curve.

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