Hubble Observations Indicate Massive Planets May Form Quickly Out In The Cold
Don Savage
Headquarters, Washington, D.C.
(Phone: 202-358-1547)
Bill Steigerwald
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 301-286-5017)
RELEASE: 00-03
HUBBLE OBSERVATIONS INDICATE MASSIVE PLANETS MAY FORM QUICKLY OUT IN THE COLD
A young star may be forming massive planets much earlier and at greater distances than current planet
formation models predict, according to new observations from the Hubble Space Telescope. NASA
astronomers using the Space Telescope Imaging Spectrograph (STIS) instrument on board Hubble
discovered that the disk of gas and dust surrounding a young star has a gap, possibly caused by
gravitational influence of a nascent planet.
The gap appears about 30 billion miles from the star, more than seven times the distance from the Sun
to Pluto, the most remote planet in our solar system. The STIS instrument also revealed that the star is
ejecting jets of gas at hundreds of miles per second from its poles, a feature usually seen in much
younger stars.
“If the clearing in the disk is due to planet formation, it suggests we have a lot more to learn about how
planets form, but we may be on the right track, which is very exciting,” said Dr. Carol Grady of NASA’s
Goddard Space Flight Center, Greenbelt, Md. “The potential planet is much further away from its star
than any known bodies in the plane of our solar system, and it is forming much faster than most models
predict.”
“This star is also interesting because of its jets,” adds Dr. David Devine of Goddard. “Protostellar jets
are a byproduct of the accretion of material onto a young star, and are thought to have lifetimes of a
few hundred thousand years or so. It came as a big surprise to find them associated with a 4 million
year old star. One possible explanation for these ‘old’ jets is that the formation of planets in the disk
results in periodic ‘meteor showers’ of material falling onto the central star, which rejuvenates the jets. In
simpler terms, the star gets the hiccups while eating dessert.”
Grady and Devine will present their results at the winter meeting of the American Astronomical Society
(AAS) in Atlanta, Georgia, Jan. 12, and the research will be published in the Astrophysical Journal.
The star, designated HD163296, is approximately 400 light years away from Earth in the direction of the
constellation Sagittarius (one light year is almost six trillion miles). It is about twice as massive as the
Sun and has an estimated age of 2-10 million years. This is relatively young, as stars of its type have
lives spanning up to one billion years.
If the gap in the disk is due to a single planet, the planet has an estimated mass 1.3 times that of
Saturn. It is likely that the planet will gain additional mass as it continues to pull material from the disk.
It would have been very difficult to detect the jets and the gap in the disk without the use of a
coronagraph. Normally, a young star’s bright light prevents astronomers from seeing material that is
close to it (imagine a match next to a spotlight). However STIS has a coronagraph that blocks the
star’s light and allows the study of the much fainter surrounding material. Unfortunately, the inner part
of the disk can’t be directly seen, because it is obscured by the coronagraph. However the astronomers
were able to trace the disk to within about 180 AU of the star (1 AU is the distance from the Earth to
the Sun, about 93 million miles (150 million kilometers) ).
Structures in the jets provide hints of what is happening in the inner disk blocked from view by the
coronagraph. “We do see dense clumps in the jet,” said Devine. “They are regularly spaced, and appear
to be ejected about once every five years.”
“One exciting, but speculative, possibility is that there is another planet forming in the inner part of the
disk,” said Grady. “As the planet progresses in its orbit, it may periodically disrupt the disk and toss
material on the star, some of which is ejected as denser clumps of material in the jets.”
“The five-year spacing of the knots in the star’s jets may be related to a five-year orbital period for any
potential companion object,” said Grady. “This is in the habitable zone for the star, where liquid water
could exist. However, since the system is young and the object is still forming, there is likely a constant
rain of meteorites on its surface. I wouldn’t want to spend a vacation there.”
“On the other hand, the knots in the jets may have nothing to do with planet formation, and may be due
to some unobserved feature of the star or disruptions caused by a low-mass companion like a brown
dwarf. A brown dwarf is an object not quite massive enough to shine by nuclear fusion like a star but
instead glows dimly with heat left over from its formation. If the companion were a low-mass star, it
should produce X-rays, but other observatories sensitive to X-rays have not seen any nearby. However,
we can’t yet rule out a brown dwarf,” said Grady.
“We have only recently had observatories powerful enough to tell us something about planet formation,
so the field is very young,” said Grady. “We don’t yet know the full diversity of solar systems — if a star
with fast, remote planet formation and jets is common or rare. Recent observations of stellar disks,
announced at last January’s AAS meeting, indicate that at least one other star (HD141569) may have
planets forming rapidly at great distances. If this were a rare occurrence, we would not expect to find
two examples so quickly.”
“We will use Hubble to take longer exposures of HD163296 in the summer to see if there is anything else
going on in its faint, outer disk, and we plan similar observations of 8-10 other stars by September. We
are very fortunate to be exploring this new frontier, and are tremendously excited by future
observatories, such as the Space Interferometry Mission and the Terrestrial Planet Finder, that promise
to tell much more,” said Grady.
In addition to taking pictures, STIS separates light into its component colors, much like a prism
separates white light into a rainbow. It turns out that each element can only emit light of certain colors,
so by analyzing the light emitted by an object astronomers can figure out its composition. Astronomers
can also learn about its motion because light from an object that is moving toward us is shifted to more
energetic, bluer colors, and light from an object moving away is shifted to less energetic, more red
colors. The effect is similar to the way the pitch of the siren on a speeding ambulance appears to rise as
it approaches and fall as it rushes away.
IMAGE CAPTION: [ftp://pao.gsfc.nasa.gov/newsmedia/aas/hst/aas_hst.jpg]
HD 163296 is an isolated young star, seen against a background of fainter stars in the plane of the
Galaxy. Material close to the star can only be seen during man-made eclipses with the star blocked from
our direct view by a coronagraph.
The figure on the left shows a composite image formed from three coronagraphic observations of HD
163296 obtained with the Hubble Space Telescope and the Space Telescope Imaging Spectrograph
(STIS), centered on the star and covering 1/2 percent of the diameter of the full moon on the sky
(20″x20″). The image is shown with north up and east to the left. The dark, irregularly shaped polygons
are regions which were obscured by the STIS coronagraph in all three observations. The disk surrounding
the star is the oval structure resembling Saturn’s rings running from lower left to upper right (SE to NW)
and is shown in false color (white is bright, with yellow to red and finally black corresponding to fainter
signals), with the very high dynamic range image displayed as the human eye perceives brightness
(logarithmic stretch). The disk extends to 450 times the Earth-Sun distance. All of the material that we
can see is at a distance well beyond the orbit of Pluto and the known size of our Solar System’s Kuiper
Belt.
The disk exhibits a surprising amount of structure. There is a bright dust ring at 350 times the Earth-Sun
distance which can be seen on either side of the star’s location, together with a darker lane just inside it.
The dark lane is 0.4″ or 50 times the Earth-Sun distance across. If this lane is cleared by a single body,
the inferred mass of the body is 1.3 Saturn masses. Inside the dark lane the disk becomes as bright as
the outer bright ring, but does not continue to brighten with decreasing distance from the star. This
suggests that there may be a region between 300 and 180 times the Earth-Sun distance which is
partially cleared. The coronagraphic image also reveals three regions of nebulosity (the fuzzy, bright
features) aligned perpendicular to the disk. These features were completely unexpected in a star this old,
and are more typically detected in association with really young protostars which are 10 times younger
than HD 163296.
The middle figure shows a spectral image of the vicinity of the star in the light of glowing hydrogen gas.
In this image, we sample material along a 0.2″ (24 times the Earth-Sun distance) wide slit passing
through the nebulosities (vertical axis of the middle image). Along the horizontal axis we see the light of
the star spread out into its constituent colors. The nebulosities above the star (NE in the coronagraphic
image) are displaced toward the red, indicating that the gas is moving away from us at velocities of 250
miles/second (400 km/s). Below the star, the data reveal a jet of material moving toward us at 250
miles/second (400 km/s). This velocity information enables us to determine the three-dimensional space
orientation of the disk and jet system. A cartoon combining the information from the coronagraphic
image and the spectrum is shown on the right. The nebulosities seen in both the image and the spectrum
have been identified as ejected gas features termed Herbig-Haro objects and collectively are cataloged
as HH409.
Photo credits: NASA and C.A. Grady and David Devine (NOAO, NASA Goddard Space Flight Center), B.
Woodgate and R. Kimble (NASA Goddard Space Flight Center), F.C. Bruhweiler and A. Boggess (Catholic
University of America), J.L. Linsky (JILA, University of Colorado and NIST), P. Plait (Advanced Computer
Concepts), M. Clampin and P. Kalas (Space Telescope Science Institute).
For high resolution images of the composite picture, go to:
ftp://pao.gsfc.nasa.gov/newsmedia/aas/hst
SCIENCE BACKGROUND:
HD 163296 is one of the brighter and better studied 2 solar mass young stars, and has an estimated
age of 2-10 million years, with a most probable age estimate of 4 million years. The HD 163296 system
shows features more commonly associated with older, planetary systems, and with younger protostars.
Previous ground-based millimeter observations had shown that the star had a large gas and dust disk,
while space-based and ground-based infrared observations had indicated that the disk material
extended in to very close to the star, much further in than we can image. Dark lanes in a dust disk have
previously been observed in a somewhat older (10 Million years) star, HD 141569 (STScI-PR99-03, which
can be viewed at
oposite.stsci.edu/pubinfo/pr/1999/03/index.html), and not in younger systems such as AB Aur
(STScI-PR99-21 which can be viewed at
oposite.stsci.edu/pubinfo/pr/1999/21/index.html) at 2-4 million years. The STIS data indicate that
features interpreted as the effects of planet formation occur sooner than predicted by models, and at
larger distances from the star.
The movement of the nebulosities between the time of the coronagraphic images and the spectral
observations suggests that the Herbig-Haro objects are moving in the plane of the sky with velocities of
approximately 190 miles/second (300 km/s), and that the spacing between the brighter knots
corresponds to ejections separated by approximately 5 years. Such a spacing could be due either to
stellar activity cycles, which have not previously been known to occur in a star of HD 163296’s type, or
possibly due to the motion of a companion orbiting the star with a 5 year period. Such a period would
place the companion within the habitable zone, where liquid water can be present on a planetary surface,
for a star of HD 163296’s temperature.
In principle, such a companion could be a low-mass star, a brown dwarf, or a planet. If the companion
were a low-mass star, it should produce X-rays, but other observatories sensitive to X-rays (ROSAT)
have not seen any X-ray emission from the vicinity of the star. This suggests that any companion must
be either a brown dwarf, an object not quite massive enough to shine by nuclear fusion, but which glows
dimly with heat left over from its formation, or a planet. The presence of structure further out in the disk
suggesting the presence of at least one planetary mass body suggests that the inner companion is more
likely to be a planet. If correct, HD 163296 may be the youngest known example of a multiple-planet
system and indicates that planets form both faster and over a wider zone in their natal circumstellar
disks than expected.
Where else has Hubble seen hints of planet formation?
http://oposite.stsci.edu/pubinfo/pr/1999/03/index.html
http://oposite.stsci.edu/pubinfo/pr/1999/21/index.html
More about Hubble:
More about the Space Telescope Imaging Spectrograph instrument:
http://www.stsci.edu/hst/#stis
More about Hubble pictures:
