Scientists Develop Cheap Method for Solar System Hunt Using McDonald Observatory Telescope

University of Texas at Austin astronomers have invented an
inexpensive method to determine if other solar systems like our own exist.

Among the more than 100 stars now known to have planets, astronomers have found
few systems similar to ours. It’s unknown if this is because of technological
limitations or if our system is truly a rare configuration. The McDonald
Observatory astronomers’ novel search method uses a Depression-era telescope
mated with today’s technology.

Astronomers Don Winget and Edward Nather, graduate students Fergal Mullally and
Anjum Mukadem, and colleagues are looking for the "leftovers" of solar systems
like ours. Their method searches for the pieces of such a solar system after its
star has died, by exploiting a trait of ancient, burnt-out Suns called "white
dwarfs."

University of Texas astronomers Bill Cochran and Ted von Hippel are also
involved, along with S.O. Kepler of Brazil’s Universidade Federal de Rio Grande
dol Sul and Antonio Kanaan of Brazil’s Universidade Federal de Santa Catarina.

Astronomers know that as Sun-like stars use up their nuclear fuel, their outer
layers will expand, and the star will become a "red giant" star. When this
happens to the Sun, in about five billion years, they expect it will swallow
Mercury and Venus, perhaps not quite reaching Earth. Then the Sun will blow off
its outer layers and will exist for a few thousand years as a beautiful, wispy
planetary nebula. The Sun’s leftover core will then be a white dwarf, a dense,
dimming cinder about the size of Earth. And, most important, it likely will
still be orbited by the outer planets of our solar system.

Once a Sun-like system reaches this state, Winget’s team may be able to find it.
Their method is based on more than three decades of research on the variability
(that is, changes in brightness) of white dwarfs. In the early 1980s, University
of Texas astronomers discovered that some white dwarfs vary, or "pulsate," in
regular bursts. More recently, Winget and colleagues discovered that about
one-third of these pulsating white dwarfs (PWDs) are more reliable timekeepers
than atomic clocks and most millisecond pulsars.

These pulsations are the key to detecting planets. Planets orbiting a stable PWD
star will affect observations of its timekeeping, appearing to cause periodic
variations in the patterns of pulses coming from the star. That’s because the
planet orbiting the PWD drags the star around as it moves. The change in
distance between the star and Earth will change the amount of time taken for the
light from the pulsations to reach Earth. Because the pulses are very stable,
astronomers can calculate the difference between the observed and expected
arrival time of the pulses and deduce the presence and properties of the planet.
(This method is similar to that used in the discoveries of the so-called "pulsar
planets." The difference is, the pulsar companions are not thought to have
formed with their stars, but only after those stars had exploded in supernovae.)

"This search will be sensitive to white dwarfs which were initially between one
and four times as massive as the Sun, and should be able to detect planets
within two to 20 AU from their parent star. This means we’ll be probing inside
the habitable zone for some stars," Winget said. (An AU, or astronomical unit,
is the distance between Earth and the Sun.) "Basically, detecting Jupiter at
Jupiter’s distance with this technique is easy. It’s duck soup," he said.

Easy, but not quick. Outer planets, orbiting their stars at large distances, can
take more than a decade to complete one orbit. Therefore, it can take many years
of observations to definitively detect a planet orbiting a white dwarf.

"You need to look for a long time for a full orbit," Winget said. "A half-orbit
or a third of an orbit will tell us something’s going on there. But for a planet
at Jupiter’s distance, a half-orbit is still six years." Winget added that for
this method, "detecting Jupiter at Uranus’ distance is easier, but takes even
longer."

For the PWD planet search, Nather conceived a specialized new instrument for
McDonald Observatory’s 2.1-meter Otto Struve Telescope. He and Mukadam designed
and built the instrument, called Argos, to measure the amount of light coming
from target stars. Specifically, Argos is a "CCD photometer" — a photon counter
that uses a charge-coupled device to record images. Located at the prime focus
of the Struve Telescope, Argos has no optics other than the telescope’s
2.1-meter primary mirror. Copies of Argos are now being built at other
observatories around the world.

Mullally continues the search for planets around white dwarfs with Argos on the
Struve Telescope. He has 22 target stars, most of which were identified through
the Sloan Digital Sky Survey. When the team finds promising planet candidates
with Argos, they will follow up using the 9.2-meter Hobby-Eberly Telescope (HET)
at McDonald Observatory.

"If we find large planets orbiting at large distances, that’s a good clue that
there might be smaller planets closer in. In that case, what you do is pound
away on that target with the largest telescope you have access to," Winget said.
The HET will enable more precise timing of the PWD’s pulses, and thus be able to
pinpoint smaller planets.

This search will be able to study types of stars unable to be studied with the
doppler spectroscopy method — the most successful planet search method to date
— Winget said. Because of idiosyncrasies in the make-up of Sun-like stars, the
doppler spectroscopy method is not very sensitive in looking for planets around
stars twice as massive as the Sun. Roughly half of the stars in Winget’s study
will be white dwarfs that were originally these types of stars. For this reason,
the PWD study at McDonald can be instrumental in scouting and assessing targets
and observing strategies for NASA space missions planned in the next two
decades, specifically the Space Interferometry Mission, Terrestrial Planet
Finder and Kepler spacecraft.

This research is funded by a NASA Origins grant, as well as an Advanced Research
Project grant from the State of Texas. Through funding from the Texas Higher
Education Agency, two secondary schoolteachers (Donna Slaughter of Stony Point
High School in Round Rock, Texas, and Chris Cotter of Lanier High School in
Austin) have been directly involved in this research. Plans are now underway to
extend this involvement to other teachers, and the students in their classrooms
by bringing the science, scientists and the Observatory directly into the
classroom using the Internet. Cotter and his colleagues at Lanier High School
are involved with Mullally in testing this concept.