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A Triple Planetary System and More

Finding three planets around a single star is newsworthy in itself, but when the planets are Neptune-class things get more interesting. And when one of these worlds is found to be in the star’s habitable zone, Centauri Dreams definitely drops everything for a closer look. Not only that, but the system around HD 69830, a Sun-like star some 41 light years away, is also the home of an asteroid belt, making the comparison with our Solar System that much closer.

Here’s what we know, as reported in a paper in the May 18 Nature: The orbital periods of the three planets are 8.67, 31.6 and 197 days, with that outer world located near the inner edge of the zone where liquid water could exist. In terms of mass, this planet is not Earth-like; in fact, the measurements show the new planets to be between 10 and 18 times the mass of Earth. So what we’re probably detecting in the habitable zone is a planet with a rocky/icy core surrounded by a dense atmosphere. We know nothing, of course, about possibly habitable moons.

A rocky composition is suggested for both inner planets as well. In terms of distance, the three worlds are 0.08, 0.19, and 0.63 AU from their star, but the location of the asteroid belt is unknown. The possibilities are between the two outermost planets or farther out than 0.8 AU. Remember that this work was all accomplished by radial velocity investigations, and much remains to be learned as we untangle the results of these measurements.

What grand work this is. The researchers, led by Michel Mayor and Christophe Lovis (Geneva Observatory) used the European Southern Observatory’s HARPS (High Accuracy Radial Velocity Planet Searcher) spectrograph mounted on the 3.6-meter La Silla instrument in Chile. HARPS has demonstrated a remarkable long-term precision of 1 m/s. Take a look at the image below to get an idea of how its data are manipulated.

Radial velocity measurements of HD 69830

Image: The HARPS radial velocity measurements of HD 69830 are folded with the orbital periods of the three discovered planets: 8.67, 31.6 and 197 days, respectively. In each case, the contribution of the two other planets has been subtracted. The solid line shows the best fit to the measurements, corresponding to minimum masses of 10.2, 11.8 and 18.1 Earth masses. Note that the full span of the vertical axis is only 13 m/s! Error bars indicate the accuracy of the measurements. The integration time was 4 minutes on average for the first 18 measurements (shown as open dots), and was increased to 15 minutes for the remaining points (full dots). The latter measurements are therefore of much higher quality. Credit: European Southern Observatory.

Centauri Dreams‘ take: What astronomers can do with the HARPS instrument is simply mind-boggling, as I am reminded every time I look into its operations. Radial velocity measurements depend on our detecting how the presence of a planet exerts a pull on its star. The velocity variations are tiny, in this case between 2 and 3 meters per second, roughly the speed of a brisk walk. Images like the one above aren’t glamorous — they don’t show us great, ringed planets or green terrestrial Earths — but the story they tell is spectacular. I often write about the breakthroughs we’ll make by imaging exoplanets directly and analyzing their spectra, but let’s not forget the continuing excellence of radial velocity research, or its ever-higher levels of sensitivity.

The paper is Lovis, Mayor, Pepe et al., “An extrasolar planetary system with three Neptune-Mass Planets,” in Nature 441 (18 May 2006), pp. 305-309.

Comments on this entry are closed.

  • anthonares May 18, 2006, 9:28

    This is a truly great find. I’m amazed with each new discovery not only at the precision of the technology that is used, but also by how the astronomy community itself has reacted to Exoplanet research. When I started my undergraduate astronomy degree in 2000 (I graduated in 2004), we were advised not to go into exoplanet research because there wasn’t much money there. But now, it’s very high profile research, despite the fact that it hasn’t yielded a whole lot of new information in how planetary systems evolve. Right now, it’s just collecting data for the most part. So much of that is due to public perception of the importance of these issues. I think that feeback is fantastic, as is your coverage of this area of research!

  • Administrator May 18, 2006, 11:30

    Much appreciated! As you can tell, exoplanetary research has become something of a passion at this end. And yes, it’s great to see that public awareness of this work continues to grow. Truly exciting stuff lies just ahead.

  • ozne May 18, 2006, 20:49

    Worlds up to 10 Me are candidates for being giant ocean planets as described in this New Scientist article :

    Unfortunately it is for subscribers only (like me) , but it contains a link to the original research that is public :


  • qraal May 19, 2006, 8:05

    Hi All

    The Ocean Planet papers are available online from a number of web-sites. One possible formation route is for Neptune-class gas-giants to be stripped of their H/He envelopes by hydrodynamic escape. This should be fairly efficient within 1 AU of a Sun-like star.

    Problem with such huge masses of water is that most of it will be high pressure phases of ice – above about 22,000 bar pressure that means Ice VII. Unless this ice-mantle is kept hot enough to undergo solid-state convection then a lot of minerals and geochemical sources of biological energy will be trapped.

    Habitable moons are a possibility if they can hang onto an atmosphere. Tidal forces might be enough to keep a magnetic field going in fairly low mass moons (eg. Io and Ganymede) and gravity a bit stronger than Mars should hang on to gases like CO2. Whether any exist is totally beyond current techniques – unless the planets are transiting their star.


  • ljk March 2, 2007, 11:46

    Astrophysics, abstract

    From: Christophe Lovis [view email]

    Date: Thu, 1 Mar 2007 16:45:33 GMT (575kb)

    An extrasolar planetary system with three Neptune-mass planets

    Authors: C. Lovis, M. Mayor, F. Pepe, Y. Alibert, W. Benz, F. Bouchy, A.C.M. Correia, J. Laskar, C. Mordasini, D. Queloz, N.C. Santos, S. Udry, J.-L. Bertaux, J.-P. Sivan

    Comments: 17 pages, 3 figures, preprint of the paper published in Nature on May 18, 2006

    Over the past two years, the search for low-mass extrasolar planets has led to the detection of seven so-called ‘hot Neptunes’ or ‘super-Earths’ around Sun-like stars. These planets have masses 5-20 times larger than the Earth and are mainly found on close-in orbits with periods of 2-15 days.

    Here we report a system of three Neptune-mass planets with periods of 8.67, 31.6 and 197 days, orbiting the nearby star HD 69830. This star was already known to show an infrared excess possibly caused by an asteroid belt within 1 AU (the Sun-Earth distance). Simulations show that the system is in a dynamically stable configuration. Theoretical calculations favour a mainly rocky composition for both inner planets, while the outer planet probably has a significant gaseous envelope surrounding its rocky/icy core; the outer planet orbits within the habitable zone of this star.