Do missing Jupiters mean massive comet belts?
In a new Herschel study, two nearby planetary systems have been found to host vast amounts of cometary debris.
November 28, 2012
Using the European Space Agency’s (ESA) Herschel Space Observatory, astronomers have discovered vast comet belts surrounding two nearby planetary systems known to host only Earth-to-Neptune-mass worlds. The comet reservoirs could have delivered life-giving oceans to the innermost planets.
Artist impression of the debris disk and planets around the star known as Gliese 581, superimposed on Herschel PACS images at 70, 100, and 160 micrometer wavelengths. The line drawing superimposed on the Herschel image gives a schematic representation of the location and orientation of the star, planets, and disk, albeit not to scale. The black oval outline sketched onto the Herschel data represents the innermost boundary of the debris disk; the approximate location of the outermost boundary is represented by the outer set of dashed lines. It is not possible to identify the central star due to smearing of the Herschel data. // Credit: ESA/AOES
In a previous Herschel study, scientists found that the dusty belt surrounding nearby star Fomalhaut must be maintained by collisions between comets.
In the new Herschel study, two more nearby planetary systems — GJ 581 and 61 Vir — have been found to host vast amounts of cometary debris.
Herschel detected the signatures of cold dust at –392° Fahrenheit (–200° Celsius), in quantities that mean these systems must have at least 10 times more comets than in our solar system’s Kuiper Belt.
GJ 581, or Gliese 581, is a low-mass M dwarf star, the most common type of star in the galaxy. Earlier studies have shown that it hosts at least four planets, including one that resides in the “Goldilocks Zone” — the distance from the central sun where liquid surface water could exist.
Two planets are confirmed around G-type star 61 Vir, which is just a little less massive than our Sun.
The planets in both systems are known as “super-Earths,” covering a range of masses between two and 18 times that of Earth.
Interestingly, however, there is no evidence for giant Jupiter- or Saturn-mass planets in either system.
The gravitational interplay between Jupiter and Saturn in our solar system is thought to have been responsible for disrupting a once highly populated Kuiper Belt, sending a deluge of comets toward the inner planets in a cataclysmic event that lasted several million years.
“The new observations are giving us a clue: They’re saying that in the solar system we have giant planets and a relatively sparse Kuiper Belt, but systems with only low-mass planets often have much denser Kuiper belts,” said Mark Wyatt from the University of Cambridge, United Kingdom.
“We think that may be because the absence of a Jupiter in the low-mass planet systems allows them to avoid a dramatic heavy bombardment event and instead experience a gradual rain of comets over billions of years.”
“For an older star like GJ 581, which is at least 2 billion years old, enough time has elapsed for such a gradual rain of comets to deliver a sizable amount of water to the innermost planets, which is of particular importance for the planet residing in the star’s habitable zone,” said Jean-Francois Lestrade of Paris Observatory.
However, in order to produce the vast amount of dust seen by Herschel, collisions between the comets are needed, which could be triggered by a Neptune-sized planet residing close to the disk.
“Simulations show us that the known close-in planets in each of these systems cannot do the job, but a similarly-sized planet located much further from the star — currently beyond the reach of current detection campaigns — would be able to stir the disk to make it dusty and observable,” said Lestrade.
“Herschel is finding a correlation between the presence of massive debris disks and planetary systems with no Jupiter-class planets, which offers a clue to our understanding of how planetary systems form and evolve,” said Göran Pilbratt from ESA.