February 2005
A Pearl Among Stars
By: Wayne Falda
HD 209458 is a middle-of-the-pack star that for a billion years held an unseen gift for a species that one day, far in the future, would have the intelligence and the means to receive it. It would take a while for Earthlings to evolve from single-cell organisms and to develop both the intellect and technological power to look deep into the Milky Way to discover the importance of this star.
The moment arrived in 2001, when astronomers trained one of the world’s largest telescopes on HD 209458, a main sequence star located in the constellation Pegasus, 900 trillion miles—or 150 light-years—away. To their surprise, they found a planet amazingly different from the planets in our solar system. The planet—HD 209458b—orbits its star every 3.5 days and is nearly the mass of Jupiter.
As is typical of nature, new discoveries require reexamination of standard theories. The age of extrasolar planet discovery in the mid-1990s abruptly ended the debate about the existence of other planets in the universe. Over 120 planets had been detected by the middle of 2004. Before then, it was thought that Jupiter-size planets could not form close to stars.
What makes the planet orbiting HD 209458 especially interesting is that its orbit is perfectly aligned such that it transits between the star and Earth. A transit occurs when a planet travels in front of the star and blocks out a fraction of the star’s surface. As we observe a planet in transit, we are offered a splendid opportunity to view its atmosphere in all its glory. Understanding the planetary atmosphere is critical to provocative questions such as: How do planets form? What are their compositions? When do they form?
The Keck II telescope and spectrograph on Mauna Kea, Hawaii, was exactly what was needed to acquire the required infrared measurements. The Keck II, one of the world’s most technologically advanced telescopes, saw its first light in 1996. Along with its twin, the Keck I, Keck II stands eight stories tall. Its 10-meter primary mirror, composed of 36 hexagonal segments, stands as one of the greatest achievements of astronomy. Each twin weighs 300 tons. Each can probe billions of years back in time and take measurements with nanometer precision.
In late 2003, Notre Dame astrophysicists submit a bold proposal to NASA. They requested three nights on Keck II to search for two specific molecules in the atmosphere of HD 209458’s planet. They believed the results would provide a better understanding of the planet’s atmosphere and might provide the data scientists needed to understand planet formation.
The proposal was accepted by the NASA review committee. In June and July 2004, two young Notre Dame scientists, Sean Brittain and Joe “Reece” Haywood, traveled with their academic advisor, Professor Terrence Rettig, and visiting professor Erika Gibb, to the 14,000-foot summit of Mauna Kea. There they would train the Keck II on HD 209458 and its planet in hopes of collecting the data that just might help answer some of the newest and most controversial astrophysical questions.
The planet HD 209458b is a gas giant very much like Jupiter, but its atmosphere is much more extended. It whirls extremely close to its parent star in what amounts to a volatile and fiery tango, all the while spewing prodigious amounts of its atmosphere into space.
“The orbital period, mass, density and radius of the planet, obtained via radial velocity and photometry measurements, are fundamental pieces of information but provide no clue as to the composition and chemistry of its atmos-phere,” Brittain says, “and that’s where this project begins.”
The objective was an audacious one. As advanced as the Keck II is, the Notre Dame group would be pushing its instruments to the limit. Their proposal was to use the worlds’ largest telescope and most advanced infrared spectro-graph to search for signatures of mole-cules expected in the planet’s atmosphere.
The observations would require extraordinarily high signal-to-noise measurements. The potential for detecting minute absorption features less than 0.1 percent deep in the transiting planet’s atmosphere would enable a level of scientific understanding of an extrasolar planet never before possible. The results would provide constraints as to how such a planet may have been formed and, if successful, would be an important new tool to probe the atmospheres of other extrasolar planets.
Other astronomers had measured sodium in the atmosphere of HD 209458b. But the Notre Dame astronomers proposed to do something no one had tried before. They wanted to home in on the fundamental wavelengths corresponding to methane as well as an ionized hybrid of hydrogen called H3+. The methane would only survive in the inner layer of HD 209458b’s atmosphere, while H3+, a more strongly bound molecule, would form in the outer regions of the planet’s atmosphere. The results would provide direct evidence as to the chemistry and structure of the atmosphere and be used to constrain the formation history of the planet.
“We are just in the early stages of under-stand-ing planet formation, even for our own solar system. No one has a theory that doesn’t have its critics,” Rettig explains.
It is known that the collapse of a disk of material into a protostar is a relatively rapid and turbulent process. In terms of a stellar lifetime, there is only a short time before the disk of material is cleared. “So we know that planets have to form within the first few million years of this process or miss their chance at being formed at all,” Rettig explains. But whether they are formed gradually or suddenly is unclear. Further complicating matters is how Jupiter-size planets are formed so close to a star.
Observations of HD 209458b would shed some light on this problem. Since we know the mass and diameter of the planet, we know its average density. It turns out that this planet is much “puffier” than scientists expected, leading scientists to suggest that the planet must have formed very warm and very close to the star. This poses a major challenge to the two main models of planet formation (see sidebar); however, more specific information about the atmosphere is necessary in order to consider specific alternatives.
Brittain thinks the Keck II’s spectrographic capabilities just might be up to the task.
“What we want to deduce from our obser-vations is the thermodynamic structure of the planet,” he explains. Did the planet form close to the star or are there other heating mechanisms at work? “Nobody knows,” Brittain says, but his team’s observations have the potential to shed light on such questions.
The Notre Dame astronomers traveled to the Keck II for their first nights of observation on June 8 and 15. When they arrived, they were in for a shock. “The Keck’s filter wheel was stuck,” Brittain says. The device that filters out unwanted infrared wavelengths was locked. The Notre Dame scientists had to settle for having the filter tune into a different wavelength. Luckily, this wavelength range allowed them to look for the fainter overtone lines of the same molecules they had chosen in their original proposal to NASA.
It was a longshot, Brittain explains. The signals they were seeking would be so faint, so weak, that it was conceivable that their best efforts could fail.
Puffy cumulus clouds almost always build around the lower elevations of Mauna Kea. But at 14,000 feet, half of the Earth’s atmosphere lies below the Keck I, Keck II, and the 11 other telescopes that dot the summit. At the top, the air is rare and dry an average of 325 nights a year. Astronomers can be reasonably certain they will not be hindered
by weather.
On June 8 and 15, viewing conditions were spectacular. The group began the pain-staking process of analyzing the data, a task which fell to Haywood, a recent graduate of Kings College in Bristol, Tenn., who came to Notre Dame in 2001 to obtain his Ph.D. in physics. The team hoped the haystack of numbers would provide clues about H3+, methane, and how this planet came to be.
To obtain the highest quality signal, the spectra must be extracted from the data in such a way that no noise is introduced. The spectra are cleaned of cosmic ray hits and calibrated with data processing software developed mainly by the group. Since the signal from the planet moves throughout the night, each spectrum (there are over 500 in all) is shifted so that the H3+ lines from the atmosphere of the exoplanet are aligned.
Haywood’s efforts to analyze the data contin-ued right up to July 30, as the team prepared for their third and final turn with the Keck II.
On the morning of the 30th, Haywood announced that he had teased out a signal that seemed to stand out from the background noise.
“It looks like the signal is pretty much where it ought to be,” Haywood said.
“This is the most promising news yet,” Brittain said as the three astronomers drove to the top of Mauna Kea to meet with the crew who operate both Keck telescopes.
Cirrus clouds above the Keck II deeply con-cerned the three. But by sundown the skies were clear for viewing. At 9 p.m. HD 209458 rose in the eastern sky and operators of the Keck II aimed the telescope directly at it.
HD 209458’s planet began its three-hour trek across the surface of the star in full view of the Keck II’s giant light-gathering mirrors. Watching HD 209458b passing in front of
its star on three occasions gave the astronomers sufficient data, but it would take months
of analysis by Haywood to locate unequivocal evidence of methane and H3+. Searching
for such faint signals requires extraordinarily careful analysis.
The Notre Dame astronomers plan to return to the Keck II again in November—their fifth visit in 2004. These next observations will concentrate on the group’s primary objective: understanding how disks around young stars form and eventually produce planets from the roiling and turbulent gases and elements forged from other dead stars.
By then Brittain will be working at the National Optical Astronomy Observatory in Tucson, Ariz., having received a prestigious Michelson Fellowship. This fellowship is awarded to recent Ph.D. graduates who are studying planet formation and/or detection.
The Keck telescopes are used primarily by astronomers at Caltech, the University of Hawaii, and the University of California. Only 32 nights a year are available for other astronomers in the U.S. The Notre Dame program has progressed over the last few years such that they have received three nights this year for themselves and an additional five to six nights a year with other collaborators. These opportunities put ND astronomers in a strong position to make important contributions to cutting-edge research and points to a strong future for Notre Dame astronomy.
Contact Wayne Falda at Wayne.Falda.1@nd.edu