Astronomy and Astrophysics – Astrophysics
Scientific paper
Aug 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004eso..pres...22.&link_type=abstract
ESO Press Release, 08/2004
Astronomy and Astrophysics
Astrophysics
Scientific paper
ESO HARPS Instrument Discovers Smallest Ever Extra-Solar Planet
Summary
A European team of astronomers [1] has discovered the lightest known planet orbiting a star other than the sun (an "exoplanet").
The new exoplanet orbits the bright star mu Arae located in the southern constellation of the Altar. It is the second planet discovered around this star and completes a full revolution in 9.5 days.
With a mass of only 14 times the mass of the Earth, the new planet lies at the threshold of the largest possible rocky planets, making it a possible super Earth-like object. Uranus, the smallest of the giant planets of the Solar System has a similar mass. However Uranus and the new exoplanet differ so much by their distance from the host star that their formation and structure are likely to be very different.
This discovery was made possible by the unprecedented accuracy of the HARPS spectrograph on ESO's 3.6-m telescope at La Silla, which allows radial velocities to be measured with a precision better than 1 m/s. It is another clear demonstration of the European leadership in the field of exoplanet research.
PR Photo 25a/04: The HARPS Spectrograph and the 3.6m Telescope PR Photo 25b/04: Observed Velocity Variation of mu Arae (3.6m/HARPS, 1.2m Swiss/CORALIE, AAT/UCLES) PR Photo 25c/04: Velocity Variation of mu Arae Observed by HARPS (3.6m/HARPS) PR Photo 25d/04: "Velocity Curve" of mu Arae
A unique planet hunting machine
ESO PR Photo 25a/04
ESO PR Photo 25a/04
The HARPS Spectrograph and the 3.6m Telescope
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Caption: ESO PR Photo 25a/04 represents a montage of the HARPS spectrograph and the 3.6m telescope at La Silla. The upper left shows the dome of the telescope, while the upper right illustrates the telescope itself. The HARPS spectrograph is shown in the lower image during laboratory tests. The vacuum tank is open so that some of the high-precision components inside can be seen.
Since the first detection in 1995 of a planet around the star 51 Peg by Michel Mayor and Didier Queloz from the Geneva Observatory (Switzerland), astronomers have learned that our Solar System is not unique, as more than 120 giant planets orbiting other stars were discovered mostly by radial-velocity surveys (cf. ESO PR 13/00, ESO PR 07/01, and ESO PR 03/03).
This fundamental observational method is based on the detection of variations in the velocity of the central star, due to the changing direction of the gravitational pull from an (unseen) exoplanet as it orbits the star. The evaluation of the measured velocity variations allows to deduce the planet's orbit, in particular the period and the distance from the star, as well as a minimum mass [2].
The continued quest for exoplanets requires better and better instrumentation. In this context, ESO undoubtedly took the leadership with the new HARPS spectrograph (High Accuracy Radial Velocity Planet Searcher) of the 3.6-m telescope at the ESO La Silla Observatory (see ESO PR 06/03). Offered in October 2003 to the research community in the ESO member countries, this unique instrument is optimized to detect planets in orbit around other stars ("exoplanets") by means of accurate (radial) velocity measurements with an unequalled precision of 1 metre per second.
HARPS was built by a European Consortium [3] in collaboration with ESO. Already from the beginning of its operation, it has demonstrated its very high efficiency. By comparison with CORALIE, another well known planet-hunting optimized spectrograph installed on the Swiss-Euler 1.2-m telescope at La Silla (cf ESO PR 18/98, 12/99, 13/00), the typical observation times have been reduced by a factor one hundred and the accuracy of the measurements has been increased by a factor ten.
These improvements have opened new perspectives in the search for extra-solar planets and have set new standards in terms of instrumental precision.
The planetary system around mu Arae
The star mu Arae is about 50 light years away. This solar-like star is located in the southern constellation Ara (the Altar) and is bright enough (5th magnitude) to be observed with the unaided eye.
Mu Arae was already known to harbour a Jupiter-sized planet with a 650 days orbital period. Previous observations also hinted at the presence of another companion (a planet or a star) much further away.
The new measurements obtained by the astronomers on this object, combined with data from other teams confirm this picture. But as François Bouchy, member of the team, states: "Not only did the new HARPS measurements confirm what we previously believed to know about this star but they also showed that an additional planet on short orbit was present. And this new planet appears to be the smallest yet discovered around a star other than the sun. This makes mu Arae a very exciting planetary system."
"Listening" to the star
ESO PR Photo 25b/04
ESO PR Photo 25b/04
Observed Velocity Variation of mu Arae
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ESO PR Photo 25c/04
ESO PR Photo 25c/04
Velocity Variation of mu Arae Observed by HARPS
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Captions: ESO PR Photo 25b/04 shows the measurements of the radial velocity of the star mu Arae obtained by HARPS on the ESO 3.6m telescope at La Silla (green triangles), CORALIE on the Swiss Leonhard Euler 1.2m telescope also on La Silla (red dots) and UCLES on the Anglo-Australian Telescope (blue circles). The solid line shows the best fit to the measurements, assuming the existence of two planets and an additional long-period companion. The fact that the line happens to have a given width is related to the existence of the newly found short period planet. The data shown span the interval from July 1998 to August 2004. ESO PR Photo 25c/04 illustrates the high-quality radial velocity measurements obtained with HARPS. Here also, the solid line shows the best fit to the measurements, assuming the existence of two planets. The data were obtained over a time span of 80 days and the first points shown are the data from the 8 nights in June. Note that the full span of the vertical axis is only 40 m/s! Error bars indicate the accuracy of the measurements. The lower part of the diagram displays the deviation of the measurements from the best fit.
ESO PR Photo 25d/04
ESO PR Photo 25d/04
Observed Velocity Variation of mu Arae
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Caption: ESO PR Photo 25d/04 displays the HARPS radial velocity measurements phase-folded with the orbital period of the newly found exoplanet (9.5 days). The measurements have been corrected from the effect of the two longer period companions. The semi-amplitude of the curve is less than 5 m/s! Coupled with the 9.5 days orbital period, this implies a minimum mass for the newly discovered planet of 14 times the mass of the Earth.
During 8 nights in June 2004, mu Arae was repeatedly observed and its radial velocity measured by HARPS to obtain information on the interior of the star. This so-called astero-seismology technique (see ESO PR 15/01) studies the small acoustic waves which make the surface of the star periodically pulsate in and out. By knowing the internal structure of the star, the astronomers aimed at understanding the origin of the unusual amount of heavy elements observed in its stellar atmosphere. This unusual chemical composition could provide unique information to the planet formation history.
Says Nuno Santos, another member of the team: "To our surprise, the analysis of the new measurements revealed a radial velocity variation with a period of 9.5 days on top of the acoustic oscillation signal!"
This discovery has been made possible thanks to the large number of measurements obtained during the astero-seimology campaign.
From this date, the star, that was also part of the HARPS consortium survey programme, was regularly monitored with a careful observation strategy to reduce the "seismic noise" of the star.
These new data
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