SETI [ASTRO] Discovery Of Extrasolar Giant Planet in Earth-like Orbit

Larry Klaes (
Thu, 29 Jul 1999 12:22:37 -0400

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>Date: Thu, 29 Jul 1999 15:50:43 GMT
>From: Ron Baalke <>
>Subject: [ASTRO] Discovery Of Extrasolar Giant Planet in Earth-like Orbit
>Reply-To: Ron Baalke <>
> Information from the European Southern Observatory
> ESO Press Release 12/99
> 29 July 1999 [ESO Logo]
> For immediate release
> ------------------------------------------------------------------------
>Extrasolar Giant Planet in Earth-like Orbit
>Discovery from a Long-term Project at La Silla
>A new extrasolar planet has been found at the ESO La Silla Observatory as a
>companion to iota Horologii (iota Hor). This 5.4-mag solar-type star is
>located at a distance of 56 light-years and is just visible to the unaided
>eye in the southern constellation Horologium (The Pendulum Clock).
>The discovery is the result of a long-term survey of forty solar-type stars
>that was begun in November 1992. It is based on highly accurate measurements
>of stellar radial velocities, i.e. the speed with which a star moves along
>the line of sight. The presence of a planet in orbit around a star is
>inferred from observed, regular changes of this velocity, as the host star
>and its planet revolve around a common center of gravity. Since in all cases
>the star is much heavier than the planet, the resulting velocity variations
>of the star are always quite small.
>The team that found the new planet, now designated iota Hor b, consists of
>Martin Kuerster, Michael Endl and Sebastian Els (ESO-Chile), Artie P. Hatzes
>and William D. Cochran (University of Texas, Austin, USA), and Stefan
>Doebereiner and Konrad Dennerl (Max-Planck-Institut fuer extraterrestrische
>Physik, Garching, Germany).
>Iodine cell provides very accurate velocity measurements
>iota Hor b represents the first discovery of an extrasolar planet with an
>ESO instrument [1]. The finding is based on data obtained with ESO's
>highest-resolution spectrograph, the Coude Echelle Spectrometer (CES) at the
>1.4-m Coude Auxiliary Telescope (CAT). While this telescope has recently
>been decommissioned, the CES instrument is now coupled via an optical fiber
>link to the larger ESO 3.6-m telescope, thus permitting the continuation of
>this survey.
>The high precision radial velocity measurements that are necessary for a
>study of this type were achieved by means of a special calibration
>technique. It incorporates an iodine gas absorption cell and sophisticated
>data modelling.
>The cell is used like an optical filter that adds its own absorption
>features to the absorption line spectrum of the star. When the radial
>velocity of a star changes, the wavelength of its spectral lines will shift
>according to the Doppler effect. They are then seen to move, relative to
>those of the iodine spectrum.
>Because of the relative nature of this measurement, the shift and hence the
>star's velocity change can be measured with a precision that is much higher
>than what the mechanical/optical stability of the spectrograph would
>otherwise allow. This particular technique is currently being applied by
>several research groups in the world and has led to most of the recent
>extra-solar planet discoveries.
>The new planet and its orbit
> [ESO PR Photo 32a/99] ESO PR Photo [ESO PR Photo 32b/99] ESO PR Photo
> 32a/99 32b/99
> [Preview - JPEG: 527 x 400 pix - [Preview - JPEG: 523 x 400 pix -
> 68k] 76k]
> [Normal - JPEG: 1053 x 800 pix - [Normal - JPEG: 1045 x 800 pix -
> 144k] 144k]
> Caption to ESO PR Photo 32a/99: Radial velocity measurements (with
> individual errors shown as bars) of the 5.4-mag solar-type star iota Hor
> over a period of nearly six years. The thin line indicates the variation
> that is caused by the new planet (as a best-fit Keplerian orbit).
> Caption to ESO PR Photo 32b/99: The combined radial velocity variations of
> iota Hor vrs. orbital phase of the newly discovered planet. The planet is
> in front of the star near phase 0.22 when the velocity is smallest and on
> the other side at phase 0.82. The orbital period is 320 days.
>For the star iota Hor, a measurement precision of about +- 17 m/sec (+- 61
>km/hour) was achieved. This is a very high accuracy in astronomical terms
>and it enabled the astronomers to detect radial velocity variations with an
>amplitude of +- 67 m/sec (or 134 m/sec peak-to-peak), cf. ESO PR Photos
>32a/99 and 32b/99.
>Five and a half years of monitoring and 95 individual spectra with exposure
>times between 15 and 30 minutes eventually revealed the presence of a
>planetary companion.
>iota Hor b has an orbital period of 320 days. From this period, the known
>mass of the central star (1.03 solar masses) and the amplitude of the
>velocity changes, a mass of at least 2.26 times that of planet Jupiter is
>deduced for the planet.
>It revolves around the host star in a somewhat elongated orbit (the
>eccentricity is 0.16). If it were located in our own solar system, this
>orbit would stretch from just outside the orbit of Venus (at 117 million km
>or 0.78 Astronomical Units from the Sun) to just outside the orbit of the
>Earth (the point farthest from the Sun, at 162 million km or 1.08
>Astronomical Units)
>The new giant planet is thus moving in an orbit not unlike that of the
>Earth. In fact, of all the planets discovered so far, the orbit of iota Hor
>b is the most Earth-like. Also, with a spectral type of G0 V, its host star
>is quite similar to the Sun (G2 V).
>iota Hor b is, however, at least 720 times more massive than the Earth and
>it is probably more similar to planet Jupiter in our own solar system. While
>the radial velocity technique described above only determines a minimum
>value for the planet's mass, an analysis of the velocity with which the star
>turns around its own axis suggests that the true mass of iota Hor b is
>unlikely to be much higher.
>A difficult case
>Natural phenomena with periods near one solar year always present a
>particular challenge to astronomers. This is one of the reasons why it has
>been necessary to observe the iota Hor system for such a long time to be
>absolutely sure about the present result.
>First, special care must be taken to verify that the radial velocity
>variations found in the data are not an artefact of the Earth's movement
>around the Sun. In any case, the effect of this movement on the measurements
>must be accurately accounted for; it reaches about +- 30 km/sec over one
>year, i.e. much larger than the effect of the new planet. In the present
>case of iota Hor, this was thoroughly tested and any residual influence of
>the Earth's motion can be excluded.
>A second complication arises from the fact that for Earth-bound telescopes,
>the visibility of a particular star changes in the course of the year. This
>creates `windows of opportunity', i.e. certain times when a given star can
>best be observed. That leads to a tendency to observe and re-observe the
>star when the planet is in the same part of its orbit. The full variation in
>radial velocity will therefore only be revealed after a sufficiently long
>time span has elapsed, covering several revolutions of the planet around the
>central star.
>More planets in the iota Hor system?
>The comparatively high scatter of the data points from the best fitting
>radial velocity curve presents an additional puzzle. While the accuracy of
>these measurements was determined as +- 17 m/sec (for other, similar stars,
>with and without known planets, an even higher precision of +- 14 m/sec was
>found with the same instrument), the scatter of the measurements around the
>mean velocity curve is higher, about +- 27 m/sec. This indicates that the
>discovered planet cannot be the whole story.
>There are two possible explanations for this additional variability.
>Either there is a second planet with another period in the same system, or
>activity on the surface of the star causes slight changes in its spectrum,
>influencing the velocity measurements.
>There are in fact indications that iota Hor is more active than the Sun,
>hence making the second explanation quite plausible. If so, it appears that
>the new planet is orbiting around a relatively young star, since such stars
>are typically more active than older ones like the Sun.
>Follow-up observations
>One of the next steps during the investigation of iota Hor will therefore be
>to get clues to its age. In any case, it is important to learn more about
>the properties of planetary orbits around young stars, in order to improve
>the theory of star and planet formation.
>After an upgrade to a spectral resolution more than twice as high as before,
>and with the larger light collecting power of the ESO 3.6-m telescope at its
>disposal, the CES spectrograph is now ready to perform direct measurements
>of the spectral instabilities that might cause the additional radial
>velocity variations in iota Hor. This project is high on the list of the
>However, they will also follow-up the other explanation - a second planet.
>Longer series of measurements may reveal additional companions in orbits
>with longer or shorter periods of revolution.
>Observations of iota Hor will therefore be continued for several more years.
>Due to its rather complex radial velocity variations, iota Hor may thus soon
>become one of the better studied stars in the southern sky.
>[1] Extrasolar Planets are also being found with the 1.2-m Swiss telescope
>at La Silla. An early success was described in ESO PR 18/98.
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