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A Question of Planets
By David F. Salisbury
December 2, 2002
How many stars like the Sun are circled by planets? Astronomers say such stars appear to be
quite rare. They base that assessment on the fact that a group of stars, called T Tauri stars, that
closely resemble the Sun when it was young appear to lose the disk of dust and gas that
surrounds them at birth before there is time for planets to form. The youngest of these T Tauri
stars clearly show such disks, but their older cousins do not. A pair of Vanderbilt astronomers
argues that the current wisdom may be wrong: Instead of loosing the basic planetary building
blocks, the material circling these stars may simply be evolving as part of the planet building
process in ways that makes it invisible to Earth’s telescopes. They have begun gathering
evidence to support their hypothesis. If they are right, then planetary systems similar to our own
may be relatively commonplace.
RESEARCH DETAILS
If David Weintraub and Jeff Bary are right, there may be a lot more planets circling stars like the
Sun than current models of star and planet formation predict.
The associate professor of astronomy at Vanderbilt and his graduate student are taking a critical
look at T Tauri stars. These are stellar adolescents, less than 10 million years old, which are
destined to become stars similar to the Sun when they mature.
Classical T Tauri stars – those less than 3 million years old – are invariably accompanied by a
thick disk of dust and gas, which is often called a protoplanetary disk because it is a breeding
ground for planet formation. Most older T Tauri stars, however, show no signs of such disks
despite the fact that they are not old enough for planets to have formed, leading astronomers to
conclude that most of Sun-like stars must loose their disk material before planetary systems can
develop.
Weintraub and Bary are pursuing a different possibility. They propose that most older T Tauri
stars haven’t lost their disks at all: The disk material has simply changed into a form that is
virtually invisible to Earth-based telescopes. They published a key observation supporting their
hypothesis in the September 1 issue of the Astrophysical Journal Letter that was highlighted by
the editors of Science magazine as particularly noteworthy. The two researchers currently are
preparing to publish additional evidence that supports their out-of-the-mainstream contention.
T Tauri stars range from about one-fifth to twice the mass of the Sun. They have been objects of
scientific interest since the discovery of T Tauri in 1852. Initially, the reason for the interest was
that their brightness varies dramatically. More recently astronomers have been studying them
because they can provide important insights into how the Sun and solar system evolved.
Twenty years ago, scientists thought that T Tauri stars had extremely strong solar winds blowing
outward at velocities of tens to hundreds of kilometers per second. The theory was based on a
spectral analysis of the light coming from these stars. One of the emission lines in their spectrum,
called the hydrogen-alpha line that is produced when protons and electrons combine to form
hydrogen atoms, is unusually strong. Astronomers figured that it must be produced by
exceptionally strong solar winds.
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A Question of Planets
In the 1990’s, however, astronomers were forced to re-evaluate this interpretation. Although T
Tauri stars may have strong stellar winds, scientists now consider the strongest source of
hydrogen-alpha emission to be hydrogen gas spiraling in from the surrounding disk to fall onto the
star. As a result, strong hydrogen-alpha lines are now considered evidence that stars possess
protoplanetary disks.
The dense disks of dust and gas surrounding classical T Tauri stars are easily visible because
dust glows brightly in the infrared region of the spectrum. Although infrared light is invisible to the
naked eye, it is readily detectably with specially equipped telescopes. The classical stars also
possess the strong hydrogen-alpha line. But there is a second group of T Tauri stars that tend to
be somewhat older – between three to six billion years – for which the hydrogen-alpha line is
either very weak or absent and which show no evidence of disks. These have been labeled
“naked” or “weak line” T Tauri stars.
Related story:
New star may hold clue to evolving planets; Register; 1995
http://www.vanderbilt.edu/News/research/ravf95/ravf95_1.html
The T Tauri stars also turn out to be strong X-ray sources. Naked T Tauri stars produce more Xray emissions than their dustier, classical cousins. So in recent years, astronomers have been
using X-ray telescopes orbiting Earth to search for them, and they’ve found hundreds.
Because the “naked” T Tauri stars do not have strong hydrogen-alpha lines and there is no visible
evidence that they possess protoplanetary disks, astronomers have concluded that they must
have lost the disk of dust and gas that they had when they were younger. The scientists argue
that this material might have been absorbed by the star or blown out into interplanetary space or
pulled away by the gravitational attraction of a nearby star.
The loss of disk material in less than 10 million years has serious consequences for planet
formation. According to current theories, it takes about 10 million years to form a Jupiter-type
planet and even longer to form a planet like Earth. If the planet-formation models are correct and
if most Sun-like stars loose their protoplanetary disks in the first few million years, then very few
stars like the Sun possess planetary systems.
This picture didn’t sit well with Weintraub, however. “Approaching it from a planetary evolution
point of view, I have not been comfortable with some of the underlying assumptions,” he says.
The focus of his dissatisfaction has been that current models do not take into account the natural
evolution that protoplanetary disks should go through as the planet-building process proceeds.
Over time, the disk material should begin agglomerating into solid objects called planetesimals.
As the planetesimals grow, an increasing amount of the mass in the disk becomes trapped inside
these solid objects where it cannot emit light directly into space. As a result, the disk material
should get progressively dimmer and more difficult to detect from a distance.
“Rather than the disk material dissipating,” says Bary, “It may simply become invisible to our
instruments.”
For his doctoral dissertation, Bary has been working with Weintraub to find ways to determine if
such “invisible disks” actually exist and can be detected even though standard methods have
failed to find them. They realized that the constituents of the disk that astronomers knew how to
detect – small grains of dust and carbon monoxide molecules – should quickly disappear during
the first steps in planet building. But the disk’s main constituent, molecular hydrogen, should stay
around much longer. The hydrogen, which makes up the bulk of the mass of giant planets like
Jupiter and Saturn, isn’t vacuumed up into the planets until rocky planetesimal cores about ten
times the size of Earth are formed.
That realization led Bary and Weintraub to search for evidence of molecular hydrogen.
Unfortunately, this form of hydrogen is notoriously difficult to stimulate into emitting light, so
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A Question of Planets
astronomers had not previously tried to look for it in the spectra from T Tauri stars. The fact that T
Tauri stars also produce X-rays gave them an idea. What if some of these X-rays were striking
the hydrogen molecules in the disk? X-rays are energetic enough to split the hydrogen molecules
into atoms, protons and electrons.
Under the proper conditions, these particles in turn could heat up the surrounding hydrogen gas
to the point that it would emit infrared radiation of a distinctive wavelength that could be detected
from Earth. Studying various theories of planet formation, they concluded that hydrogen
molecules should be present in appropriate conditions in a “flare region” near the outer edge of
the protoplanetary disk.
The next step was to get observation time on a big telescope to put their theory to the test. “That
was the hardest part,” says Weintraub, looking back. Their proposals were turned down for
several years, but they were finally allocated viewing time on the four-meter telescope at the
National Optical Astronomical Observatory in Kitt Peak, Arizona. When they finally took control of
the telescope and pointed it toward one of their prime targets – a naked, apparently diskless T
Tauri star named DoAr21 – they found the faint signal for which they were searching.
“We found evidence for hydrogen molecules where no hydrogen molecules were thought to
exist,” says Weintraub.
When Bary calculated the amount of hydrogen involved in producing this signal, however, he
came up with about a billionth of the mass of the Sun, not even enough to make the Moon. As
they argued in their Astrophysical Journal Letter article, they believe that what they have detected
is only the tip of the iceberg since most of the hydrogen gas will not radiate in the infrared. The
question that remains is whether the iceberg constitutes a complete protoplanetary disk or just its
shadowy remains.
Since this first discovery, Bary and Weintraub have detected the same hydrogen emission line
around three classical T Tauri stars with visible protoplanetary disks. They have found that the
strength of the hydrogen emission lines in the three is comparable to that measured at
DoAr21.They have used these results to obtain the ratio between the mass of hydrogen
molecules that are producing the infrared emissions and the mass of the entire disk in each of the
three systems. For all three they calculate that this ratio is about one in 100 million.
“If the ratio between the amount of hydrogen emitting in the infrared and the total amount of
hydrogen in the disk is about the same in the two types of T Tauri stars, which is not an
unreasonable assumption, this suggests the naked T Tauri star has a sizable but hard-to-detect
disk,” says Bary.
In one of the stars, the disk is edge-on. That allowed the researchers to measure the Doppler
shift of the region producing the infrared emissions. The shift corresponds to an orbital velocity
comparable to that of Saturn, which clearly places the location of the emitting region within the
protoplanetary disk, right where they expected it.
Weintraub and Bary admit that they have more work to do prove their theory. They have been
allocated time on a larger telescope, the eight-meter Gemini South in Chile, in order to search for
a second, fainter hydrogen emission line. If they find it, comparison of the strength of the two lines
will provide additional insights into the process that is exciting the hydrogen gas. To determine if
the hydrogen emissions that they have discovered are caused by a general mechanism involved
in the planetary formation process, the researchers also plan to survey about 50 more naked T
Tauri stars for molecular hydrogen emission lines.
Currently, the number of naked T Tauri stars that have been discovered is much greater than the
number of known classical T Tauri stars. If a significant proportion of them have kept their
protoplanetary disk, it could mean that solar systems similar to our own are a common sight in the
universe.
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A Question of Planets
Additional information
David Weintraub’s home page
http://www.physics.vanderbilt.edu/cv/weintraub_cv/frontpage.html
BIOGRAPHICAL SKETCH
David Weintraub admits that it’s a bit embarrassing when he’s on a Boy Scout camping trip with
his son and someone asks him to identify all of the constellations.
“That is what people expect from an astronomer, but I can’t do it. I never could,” he says in a tone
of wry resignation.
Most professional astronomers start out as amateur star-gazers when they are young. They build
their own telescopes and spend countless hours scouring the mysterious lights in the night sky
looking for planets, meteors and distant galaxies.
Not Weintraub. When he was a boy, an uncle gave a telescope to him and his brother. “I
remember setting it up in the front yard. We pointed it at the moon and I looked through it, but it
didn’t do anything for me.”
Weintraub was a “university brat,” however, so he didn’t lack intellectual stimulation. His father,
Stanley, who is now retired from Pennsylvania State University, is a well known historian,
biographer and authority on George Bernard Shaw. As a result, he grew up in the culturally rich
university environment and a home that placed a high value on education.
In high school Weintraub developed a passion for gymnastics rather than academics. He enjoyed
physics and math, but didn’t find them very challenging. He got approval to take an astronomy
course at the university and it was a disaster. When class began, the professor would take roll,
step up to the blackboard and begin writing and lecturing with his back to the class. When the
period was up, he took roll a second time and left, without answering any questions. “That put me
off of astronomy for some time,” he recalls.
As college approached, gymnastics was a top priority. So he searched for a university with a
strong liberal arts program, a bad gymnastics team but a really good gymnastics coach. “That
way I could be on the team even if I was not very good and would have the opportunity to get
good if I wanted to,” he says.
Yale fit the bill. Although he graduated with a bachelor of science degree in physics and
astronomy, Weintraub acknowledges that he really majored in gymnastics, spending 25 hours a
week in the gym. He became Ivy League Champion and New England Champion in several
events but never made it to the nationals.
After he graduated in 1980, Weintraub gave some thought to astronomy, but wasn’t certain it was
right for him. Instead he enrolled as a doctoral student in the department of earth and space
sciences at the University of California at Los Angeles. There he got interested in the question of
how planets form. At that time the only planet that scientists knew much about was Earth, so the
emerging discipline of planetary science was largely housed in geophysics departments like the
one at UCLA.
Weintraub spent two and a half years filling up two notebooks with equations, trying to figure out
how the gaps in the asteroid belt between Mars and Jupiter could have formed. He got his
masters degree and was on track for getting his doctorate in only four years.
Then he realized that he didn’t enjoy what he was doing and decided to take a break to think
things over. He obtained a long-term leave from the university. At that time he was into bicycling,
so he packed up his bike and headed for Europe for six months. Living on less than five dollars a
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A Question of Planets
day, he cycled across the hilly country sides of Greece and Israel, made a number of new friends
and had an extremely good time before giving the bike away and heading home.
When he returned, Weintraub landed a job teaching astronomy at Santa Monica Community
College and discovered that he enjoyed teaching. However, he also decided that he only wanted
to do this kind of teaching if he combined it with research. Consequently, when the college
president offered him a tenured position a year later, Weintraub told him he would have to think it
over, a reply that prompted the president to withdraw the offer later in the same day.
Weintraub was also thinking about becoming a high school science teacher and so he
interviewed with the Los Angeles School Department. The interviewers told him they wanted to
hire him as a high school calculus and physics teacher and gymnastics coach. “I thought this was
my dream job,” he says. When the contract arrived, however, it stated that he was being hired as
a “secondary school science teacher” and he decided not to take the job when the school officials
said they couldn’t guarantee their verbal agreement in writing.
Almost by default, Weintraub went back to graduate school. When he returned, he found that the
way astronomers thought about planets was changing. New tools were becoming available that
allowed them to study planets in new ways. NASA had launched IRAS, the first infrared
astronomical observatory. It was the beginning of the new discipline of infrared astronomy, which
extended mankind’s window on the universe beyond the bounds of visible light. Infrared
astronomy was particularly well suited for studying processes like star and planetary system
formation.
When Weintraub realized this, he went to the astronomers at UCLA to explore the possibility of
switching departments. He found an astronomer who would be happy to work with him on infrared
studies of planet formation. But he also found out that he would have to take all of the
department’s courses despite the fact he had completed all the required coursework and
advanced to candidacy in earth and space sciences doctoral program. That led him back to the
geophysicists and, with the help of one of the faculty members, he persuaded the department to
let him do his dissertation on planet formation under its auspices. Although he got what he
wanted, Weintraub became an academic orphan. “That was the hardest thing: I couldn’t even get
an all important parking permit,” he recalls.
Related story
Astronomer awarded 2001 Chancellor’s Cup; Register; 2001
http://www.vanderbilt.edu/News/register/Oct16_01/story1.html
Weintraub’s fascination with the process of planet formation carried him through these problems
and he received his doctorate in 1989. After finishing up at UCLA, Weintraub went to the
University of Florida for a post-doctoral fellowship before coming to Nashville. He chose
Vanderbilt because of its emphasis on teaching as well as research. “Vanderbilt is a perfect place
for me because, unlike most research universities, it takes teaching seriously,” he says. So, in
addition to continuing to study the process of planet formation and supervising doctoral students,
Weintraub enjoys teaching introductory astronomy, has designed an award-winning course on
science and religion and has been recognized for his efforts outside the classroom to improve the
relationship between faculty and students.
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Additional information
David Weintraub’s home page
http://www.physics.vanderbilt.edu/cv/weintraub_cv/frontpage.html
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A Question of Planets
BARY PROFILE
By Jenny Sebastian
Standing in front of a science class of twenty-one eighth graders, Jeff Bary explains that they will
be doing “all kinds of cool stuff today.” Ignoring their customary blank stares, he begins using
matches, rubber bands, clay and pieces of wood to demonstrate some basic science concepts.
As he does so, the students gradually perk up and begin asking him questions about what he is
doing.
Twice a week, Bary helps out in a science laboratory at a middle school in the Nashville area
under the auspices of a federally funded science education program. But his main job is finishing
up his doctoral thesis on planet formation under the supervision of David Weintraub, associate
professor of astronomy at Vanderbilt.
“Jeff is a very talented teacher and, in the last few years, he’s become quite energized by his
research as well,” says Weintraub.
When asked what he hopes to convey to the eighth graders, Bary explains that he hopes to give
them “some sense of what they’re capable of and maybe teach them a little science in the
meantime.”
Bary knows first hand how important it is for teachers to spark interest in children at an early age.
He grew up in a small town in West Virginia. Out of his graduating high school class of 240, less
than 10 percent went on to college. He is the first person in his family to enroll in a doctoral
program. But his father, who was a middle school science teacher, helped him overcome the
odds by instilling the value of education and encouraging him to set high educational goals.
“I went to college in spite of my education,” Bary explains with a laugh. When he was ten, he
developed an interest in astronomy, but he didn’t have the money to buy a telescope. So he
sought out and read all the astronomy books that he could get his hands on. In his high school,
the single physics course was taught by the assistant baseball coach and did little to prepare him
for the demands of college-level physics.
When it came time to go to college, financial considerations led Bary to chose a small liberal arts
school in southwest Virginia over his first choice, Vanderbilt. Unfortunately, the college did not
offer an astronomy degree, so he majored in physics and mathematics instead. After he
graduated, he wanted to go on to graduate school in astronomy, but he wasn’t accepted by any of
the schools to which he applied. So he returned to his hometown for a year to substitute teach.
It was a natural choice for Bary. During high school, he taught tennis and swimming. In college,
he tutored fellow students in mathematics and physics. So he had been teaching in one capacity
or another since he was 17 years old.
Following this break, Bary decided to enter the doctoral program in physics at Vanderbilt even
though it doesn’t offer an advanced degree in astronomy. Nevertheless, he managed to work his
way into astronomical research by linking up with Weintraub, one of the small cadre of
astronomers on campus.
Working with Weintraub, Bary has focused on the question of how long it takes planets to form.
This has given Bary the opportunity to travel to four different sites to examine a number of stars
with world-class telescopes.
The first of these trips held a particular significance for him because it allowed him to fulfill a
childhood dream. When he was in junior high school, he recalls imagining himself going to Hawaii
to study the stars. So he was particularly thrilled when he traveled to Hawaii to make
observations from two infrared telescopes situated at the summit of the Mauna Kea volcano.
Bary sees two basic reasons for scientific research and discovery.
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First, he believes that people are naturally curious. So it is only natural to wonder about the
universe around us. “It’s important to understand the origins of our solar system and [to
determine] if what’s happened around our star, the Sun, has actually happened somewhere else.”
Second, Bary argues that “anytime you attempt some scientific endeavor, there’s a need for
technology and the need ends up driving creativity and innovation.” He cites the fact that there
was no real need to have a man walk on the moon, but that this achievement led to major
improvements in a wide range of instruments and technology. And this improved technology, in
turn, has helped make the world a “smaller” place.
Bary hopes to finish his dissertation on planet formation in the spring. At the same time, he is
dedicated to doing all that he can to give his eighth graders a better sense of their own
capabilities . . . and teach them a little science in the process.
-VU Jenny Sebastian is a major in the Communications of Science, Engineering and Technology
program.
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