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Debris Belts around Vega
Topic:
Exoplanets
Concepts:
Infrared observations,
debris disks, exoplanet
detection, planetary
systems
Missions:
Spitzer, Herschel
Coordinated by the
NASA Astrophysics Forum
An Instructor’s Guide for using the
slide sets is available at the ASP
website:
https://www.astrosociety.org/
education/resources-for-the-higher-education-audience/
Vega is wearing two belts!
• Infrared observations reveal the star
Vega has a “warm” inner dust belt in
addition to the previously observed
outer “cold” dust belt.
• The discovery can be explained by two
belts of leftover planetesimals whose
ongoing collisions produce the detected
dust — analogous to the Sun’s asteroid
and Kuiper Belts. Could this also suggest
a similar planetary system for Vega?
Above: Image of the debris disk around the star
Vega taken in infrared light with the Herschel
Space Observatory. (Credit: Su et al., 2013)
Right: Artist’s illustration of debris belt around Vega.
(Credit: NASA/JPL-Caltech)
How was the discovery made?
• The Spitzer Space Telescope and the
Herschel Space Observatory detected infrared light in excess of that expected from the star alone.
• By comparing Vega with similar stars,
scientists subtracted the infrared
emission produced by the star itself,
leaving the emission radiated by surrounding dust particles warmed by
the star.
Herschel Space Observatory infrared image
of the debris disk around the star Vega, after
subtracting the light of the star Vega itself.
(Credit: Su et al., 2013)
• The inner region of infrared excess
could not be resolved into a belt like
the outer one, but the temperature
of the dust is consistent with an asteroid belt about the same proportional
distance from Vega as ours is from
our smaller Sun.
The Big Picture
• Scientists have found other stars with similar inner and outer debris belts, with the
outer belt about ten times farther from the stars than the inner belt.
• One such star,
HR8799, has been
found to have four
giant planets orbiting
in the gap between
belts.
• Our own solar
system also has four
giant planets orbiting
between the rocky
asteroid belt and the
icy Kuiper belt.
• No planets have yet been detected around Vega, but if the star were eventually
found to have several giant planets in orbit, this may suggest a common model
for how stars form planets and how their planetary systems evolve.
How does this discovery change our view?
• Other stars can host planetary
systems similar to the Sun.
The outer belts of both Vega
and Sun are about ten times
more distant from the star
than the inner belts. Our solar
system has planets between
debris belts, and Vega may
have planets in the “gap” also.
Credit: NASA/JPL-Caltech
In this diagram, the Vega system (left) is compared with our Solar
system scaled up by a factor of four (right). The inner asteroid
belt (orange) and Kuiper belt (red) are similar in relative scale,
with the distance from the stars to the outer debris belts about
ten times greater than the inner belts. The true relative size of our
solar system is illustrated by the small drawing in the middle.
• Multiple debris belts may be a
clue to the presence of several
planets.
• Our solar system may not be
unique.
Resources
Press Release about this result:
http://www.nasa.gov/home/hqnews/2013/jan/HQ_13-006_Vega_Asteroid_Belt.html
Scientific paper:
Su, et al., 2013, ApJ 763:118
http://iopscience.iop.org/0004-637X/763/2/118/
Synopsis article:
http://astrobites.org/2013/01/20/asteroid-belt-found-in-the-vega-system/
Additional resources:
Exoplanet resource guide http://www.astrosociety.org/education/astronomy-resource-guides/
the-search-for-planets-around-other-stars/
AAS announcement by Kate Su https://www.youtube.com/watch?v=neJkNoFl2uM
Extrasolar Planets Encyclopaedia http://exoplanet.eu/
Debris Belts around Vega
BONUS CONTENT
Why haven’t we found planets for Vega?
• Vega is oriented on the plane of the
sky, so we’re looking at one of its
poles, and its debris disk is seen faceon, not edge-on.
• We can’t detect planets by the
“transit” method, since they wouldn’t
pass in front of the star from our
perspective.
• We can’t detect planets by the
“Doppler” method because stars
like Vega rotate very fast and have
very few narrow lines in their spectra
Credit: NASA/JPL-Caltech
for detection of the tiny shifts in line
position caused by the gravitational tugs of planets.
• Direct visual searches have turned up empty, suggesting orbiting planets are too
small for us to detect yet by this method.
• The more powerful James Webb Space Telescope or more sensitive groundbased searches may eventually find Jupiter-sized or smaller planets orbiting in
Vega’s gap.
Collected Instructor’s Notes, part 1
“Vega is wearing 2 belts!” (page 2 in this PDF)
Debris Disk: a disk of small particles encircling a star or planet. “Belts,” “disks,” and “rings” are all debris disks.
The Figure is the Herschel view of the whole system, where the star is very bright at the center
The two solid circles mark radii of 5′′ and 7′′, while the dashed circle marks a radius of 14′′ (representing the cold planetesimal ring).
“How was the Discovery Made?” (page 3 in this PDF)
Herschel = (an ESA* mission with NASA participation)
*ESA = European Space Agency
Figure is the glowing dust image after the contribution of the star has been taken out.
Arrows point to the location of the belts.
Colors represent intensity of light, from bright (red) to faint (blue).The inner belt is not resolved into a ring, but rather is detected as excess
light at the star’s position. Vega is viewed almost face-on.
“The Big Picture” (page 4 in this PDF)
Astronomers study stars with varying characteristics to understand how planetary systems evolve.
Vega’s system is similar to our solar system, but Vega is different from the Sun.
Both stars have cold, outer belts like our Kuiper Belt (~50 K)
Vega is young(~400-600 million years old), while the Sun is 4.6 billion years old.
Vega is bright (spectral type A), 16-40 times more luminous than the Sun
The clear separation of the belts could be explained by the presence of planets clearing the gap. Indeed, HR 8799 is already known to have 4
planets in this region.
“How does this discovery change our view” (page 5 in this PDF)
Vega and the Sun all have a warm inner belt at ice/gas transition temperature (~170K) and a cold outer belt at ~50K.
The outer belt of Vega is about 10X more distant from the star than the inner belt.
The distance between our Sun’s (outer, colder) Kuiper and (warmer, inner) Asteroid belts is also a 10:1 ratio
Model calculations predict that several planets (smaller than Jupiter) maintain a dust-free zone by sweeping up material in that gap.
Thus it is likely that several yet-undetected planets reside in the gap between Vega’s belts.
Could double belts be a hallmark of a multi-planet system? – focus future observations on similar systems!
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Collected Instructor’s Notes, part 2
“Resources” (page 6 in this PDF)
Contributing Missions:
Herschel Space Observatory: http://www.nasa.gov/mission_pages/herschel/index.htmll
Spitzer Space Telescope: http://www.nasa.gov/mission_pages/spitzer/main/index.html
“Why haven’t we found planets for Vega?” (page 8 in this PDF)
Each technique is sensitive to detecting planets with certain physical characteristics (size, distance from star, inclination, etc).
Astronomers use all the evidence available to target their searches. For example, if there’s indirect evidence such as in this case, astronomers
could target direct observations.
Subtleties in why we can’t detect planets by the “Doppler” method: “early-type” stars like Vega rotate very fast and have very few narrow
spectral lines. This means it is very difficult to notice any star wobble. The dominating reason is simply line of sight.
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