Our solar system consists of a warm-and-cold-belt architecture. The warm belt corresponds to the asteroid belt and the cold belt corresponds to the Kuiper belt. Both belts are separated by a large gap that is populated by the giant planets - Jupiter, Saturn, Uranus and Neptune. The asteroid belt has a characteristic temperature of ~170 K and is located between the orbits of Mars and Jupiter. The more outlying Kuiper belt has a characteristic temperature of ~50 K and it extends beyond the orbit of Neptune. It is worth considering if such warm-and-cold-belt architectures also exist around other stars, if they might be common and if such belts may serve as clues to the existence of planets.
Spectroscopic studies were performed
using the infrared spectrograph instrument on NASA’s Spitzer space telescope on
Vega - a relatively nearby star about 25 light-years away. The observations
revealed a mid-infrared excess which corresponds to material with a
characteristic temperature of ~170 K. This indicates the presence of an
asteroid belt at a distance of ~14 AU around Vega, analogous to the asteroid
belt in our solar system. Since Vega is 37 times more luminous than our Sun,
the asteroid belt around Vega is located much further out from the star than
the asteroid belt in our solar system which is ~2.7 AU from our Sun.
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In addition to an asteroid belt, Vega is
also known to have a cold belt exterior to the asteroid belt. This cold belt is
likely to be analogous to our solar system’s Kuiper belt and it reveals itself
as an excess in the far-infrared, corresponding to material having a
characteristic temperature of ~50 K. This suggests that Vega possesses a
warm-and-cold-belt architecture like our Sun.
Observations have also revealed the
presence of hot dust in the close vicinity of Vega. The dust shows up as an
excess of short-infrared radiation corresponding to a characteristic
temperature of ~1500 K. Based on this temperature, the dust is expected to
exist within a distance of ~0.2 AU from Vega. The dust is proposed to be
nano-size metal oxides coming from the sublimation of silicate-rich
planetesimals. These nano-size metal oxides become charged either via the
photoelectric effect or the stellar wind. Once charged, they become trapped in
the star’s magnetic field at close proximity to the star.
Similar to the asteroid belt in our
solar system, the asteroid belt around Vega is also located at the water-frost
line. During the formation of planets in a protoplanetary disk, the water-frost
line demarks a boundary around a star where temperatures become cool enough for
grains of water ice to form. As a result, the density of solid particles in a
protoplanetary disk increases abruptly by a factor of a few, beginning at the
water-frost line and extending outwards. In addition to longer dynamical
timescales, the increase in the amount of solid material favours the formation
of giant planets exterior to the water-frost line. Our solar system is a good
example where 4 giant planets exist beyond the asteroid belt. Besides our Sun,
another example is a young star called HR 8799 which has a similar two-belt
configuration with 4 giant planets between the inner and outer belts. These
giant planets dominate the dynamics of the inner and outer belts, and help keep
the large gap between the two belts relatively clear.
The large gap separating the warm and
cold belts around Vega hints that Vega could be surrounded by multiple
undiscovered planets. In fact, a number of searches have been conducted to look
for planets around Vega. Although no planets have been discovered around Vega
so far, these searches have placed strong limits on the maximum mass that
planets around Vega could have. This limiting mass is no more than a few times
the mass of Jupiter. As a result, the presence of one or two very massive
planets with several times the mass of Jupiter is unlikely to be responsible
for the large gap separating the inner and outer belts around Vega. Instead,
the presence of multiple Jupiter-mass planets is sufficient to maintain the gap
and keep within the planetary mass constrains. Better observations will be
required to detect the presence of these planets.
Vega has an inner warm belt and an outer
cold belt. The two belts are separated by a large gap that is likely to be
populated by multiple planets. Besides our Sun, Vega and HR 8799, a number of
other stars are also known to posses warm-and-cold-belt architectures. These
stars include Fomalhaut and Epsilon Eridani.
Reference:
Su et al. (2013), “Asteroid Belts in
Debris Disk Twins: VEGA and FOMALHAUT”, arXiv:1301.1331 [astro-ph.EP]