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Discs06 Scientific Highlights

Last Updated: 2006-07-05

The purpose of this page is to showcase some of the most exciting results to be presented at the conference, with a particular emphasis on those which may be of interest to the general public as well as the scientific community. It will be updated regularly before and during the event, as news of new results come in. If you think a story should be added to this page, please contact Suzanne Aigrain.

Recently discovered planets should harbour fluid water (posted 05/07/2006)
Tracing giant planets during their formation (posted 05/07/2006)
Gas and dust stratification in young circumstellar disks (posted 05/07/2006)



Recently discovered planets should harbour fluid water

A few weeks ago, a team of scientists led by Christophe Lovis from Geneva observatory announced the discovery of a system containing three Neptune-mass planets (see ESO press release from 18/05). This system is particularly interesting because it shares many properties with out own, most notably the fact that it has three planets of similar size close to the central star, one of which is near the inner edge of the "habitable zone" (where liquid water can exist at the surface of rocky or icy bodies), and an asteroid belt.

Now, a team led by Yann Alibert at the University of Bern have finished a detailed investigation into the past history of this system. As illustrated schematically in the diagrams below, they have been able to show that the inner planet, initially composed of a rocky core with a thick gaseous enveloppe, should have had most of its enveloppe evaporated off by radiation from the central star as it migrated closer in. But, most interestingly, the outer two planets should have accreted a significant amount of water ice early on, then moved into warmer regions of the system. Some of the water is now expected to be in the form of a fluid at very high pressure, buried under a thick enveloppe of gas. This is the first time that extra-solar planets have been shown to contain fluid water, albeit in a form that is probably very different from the water oceans of our own world.

Schematic representation of the formation of the system, showing the inital and present day positions of the planets (numbers indicate distances from the star in astronomical units) together width the material they accreted as they migrated through the system (blue: ice, brown: rock, grey: gas) and their present day composition. The asteroid belt is also shown.

Present day state of the system, showing asteroid belt and the positions and inner composition of the planets.

Yann Alibert will give a talk entitled Extended core accretion models of planet formation on Wednesday 19th July at 12.05 pm. For more information, he can be contacted by email.

References:

  • An extrasloar planetary system with three Neptune-mass planets
    Lovis, C., Mayor, M., Pepe, F., Alibert, Y. et al., Nature, Vol. 441, p. 305 (2006)
  • Formation and structure of the three Neptune-mass planets system around HD69830
    Alibert, Y., Baraffe, I., Benz, W. Chabrier, G. et al., to appear in Astronomy and Astrophysics



Tracing giant planets during their formation

Planets are thought to form in circumstellar disks, the disks of gas and dust which naturally form around emerging young stars, and from which the latter acquire the material they are made from. While we now have a detailed picture of the evolution of these disks, exactly how planets form within them is still under discussion. To verify or rule out the various theoretical scenarios, observations of planetary systems in the process of forming are needed.

Over the next few years, a number of new large interfermoetric facilities (for example the Atacama Large Millimeter Array or ALMA) will become available. By combining the light of many separate telescopes pointing at the same object (such as a young star surroundered by a disk), they will enable us to resolve its close environment within more intricate detail than was ever possible before. Using numerical simulations, Sebastian Wolf from the Max Planck Institute for Astronomy in Heidleberg and Gennaro D'Angelo of the NASA Ames Research Center have shown that ALMA will enable us to trace young, still accreting giant planets in nearby protoplanetary disks. Because different scenarios of planet formation make very different predictions for what planetary systems should look like in their early stages, these observations will provide clues as to the mechanism(s) which actually govern planet formation.

Simulated image of a face-on circumstellar disk situated 50 parsec away from the Earth, as it would be seen by ALMA. The most readily observable signatures of the presence of a planet (whose orbit is indicated by the dhaed line) are the gap it creates in the disk and the clumb of bright, hot material near the planet itself.

Artist's impression of ALMA.

Sebastian Wolf will give a review talk entitled Prospects for the detection of protoplanets on Thursday 20th July at 2.00 pm. For more information, he can be contacted by email.

References:

  • On the Observability of Giant Protoplanets in Circumstellar Disks
    Wolf, S., D.Angelo, G., The Astrophysical Journal, Vol. 619, p. 1114 (2005)



Gas and dust stratification in young circumstellar disks

In the standard picture of star formation, stars form in giant clouds of gas and dust called molecular clouds. Under the influence of their own gravitiy, slightlly denser clumps in thes clouds collapse to form a stellar embryo, or protostar, surrounded by an spinning enveloppe of gas and dust which gradually settles into a thin circumstellar disk. The heavier materials (small rocks and dust) quickly fall to the mid-plane of the disk, while the light gaseous material remains more diffuse above and below. It is in these circumstellar disks that planets form, and the details of the settling down, or stratification process are crucial in determining excatly what type of planets can form where in the disk and how quickly.

Using the world's largest optical telescope (Keck II), a team of astronomers led by Terence Rettig of the University of Notre Dame were able to measure exactly how much gas and dust lies along the line of sight to four young stars surronded by circmustellar disks, which are very likely to be in the midst of forming planetesimals (comets), and perhaps protoplanets and planets. By comparing the relative amounts of gas and dust in the four objects, whise disks lie at different inclinations, they can tell how much dust has settled down.

These measurements allow them to deduce important information on the conditions in the disk midplane, most importantly the degree of turbulence. For a simple analogy closer to home, we can compare how bright the Sun appears at noon and at Sunset. The difference in apparent brightness is due to the number of dust particles along the line of sight through our own atmosphere, which is much larger at sunset because the Sun's rays have to cross more of the atmosphere to reach us. We would be able to tell if the earth's atmosphere were extremely turbulent (as happens in a dust storm), because then there would be far less difference between the Sun's brightness at noon and sunset.

Rettig and collaborators expect their results to be used by theorists in the ongoing efforts to understand planet formation. If the midplane of circumstellar disks is very turbulent, it is unlikely that sufficient numbers of particles can settle and stick together to form planetesimals. In the long run, understanding the degree of turbulence will enable us to constrain the rate at which dust particles of all sizes can coagulate to spur on planet growth.

Density dust grains (10 microns, top), small dust particles (0.1 micron, middle) and gas (bottom) as a function of height above the midplane and distance from the central star, in the model proposed by Rettig and collaborators.

Terrence Rettig will present a poster entitled Dust stratification in young circumstellar disks which will be displayed between Monday 17th July and Thursday 20th July. For more information, he can be contacted by email.

References:

  • Dust Stratification in Young Circumstellar Disks
    Rettig, T., Brittain, S., Simon, T., Gibb, E. et al., to appear in the July 20th 2006 issue of the Astrophysical Journal