First Light on New Planet Finder
Beginning February 11 – 27, 2003, the initial commissioning period of a planet finder–the new HARPS spectrograph (High Accuracy Radial Velocity Planet Searcher) –on the 3.6-m telescope was completed at the ESO La Silla (the ‘saddle’) Observatory bordering the southern extremity of the driest place on Earth, the Atacama desert in Chile.
|The Atacama Desert is in the north of Chile, about 1300km (800mi) from Santiago.[There are parts of Atacama where rain has never been recorded and the precious little precipitation (1cm/0.3in per year) that does fall comes from fog.]|
Michel Mayor, Director of the Geneva Observatory and co-discoverer of the first known exoplanet, is confident: "With HARPS operating so well already during the first test nights, there is every reason to believe that we shall soon see some breakthroughs in this field."
"First Light" occurred on February 11, 2003, during the first night of tests. The instrument worked well and was fine-tuned during subsequent nights, achieving the predicted performance already during its first test run.
This new instrument is optimized to detect planets in orbit around other stars ("exoplanets") by means of accurate (radial) velocity measurements with a precision of 1 meter per second. This high sensitivity makes it possible to detect variations in the motion of a star caused by the gravitational pull of one or more orbiting planets, even relatively small ones.
|The detection method known as the Doppler Shift is a ground-based method for searching for extrasolar planets.|
Credit: California & Carnegie Planet Search
The traditional ground-based method of looking for extrasolar planets uses the Doppler shift – a change in color of a star’s light to measures changes in the star’s velocity caused by the gravitational pull of an orbiting planet. This technique looks for a wobble in a star caused by the gravity of the orbiting planet. The wobble shows up as a periodic change in the spectrum of the star’s light. This change can be measured by high-resolution spectrographs mounted on telescopes. The technique can detect only Jupiter-sized planets.
The measurement of accurate stellar radial velocities is an efficient way to search for planets around other stars. More than one hundred extrasolar planets have so far been detected, providing a clear picture of a great diversity of exoplanetary systems.
Current technical limitations however have so far prevented the discovery of exoplanets around solar-type stars, if the exoplanets that are less massive than Saturn, the second-largest planet in the solar system. HARPS will break through this barrier and will carry this fundamental exploration towards detection of exoplanets with masses like Uranus and Neptune. Detecting planets much smaller than Neptune requires such precision because the velocity shifts due to the planet are masked by noise in the velocity shifts from the star itself. A further limitation of the radial velocity method is that it doesn’t allow astronomers to determine the inclination of the planetary orbit relative to Earth. As a result, only lower limits can be set for the masses of planets found using this technique.
Beginning in October 2003, the HARPS instrument will be offered to the research community in the ESO member countries.
During the first commissioning period in February 2003, the efficiency of HARPS was clearly demonstrated by observations of a G6V-type star of magnitude 8. The "G" refers to the temperature of the star (other temperature classes are O, B, A, F, K and M). A "main sequence " star is a star in the middle of its life cycle. Our Sun is a G2 (which means our Sun is hotter).
This test observation of a G6V-type star shows it is similar to, but slightly less heavy than our Sun and about 5 times fainter than the faintest stars visible with the unaided eye. During an exposure lasting only one minute, a signal-to-noise ratio (S/N) of 45 per pixel was achieved – this allows to determine the star’s radial velocity with an uncertainty of only ~1 m/s (~3 ft/s). For comparison, the velocity of a briskly walking person is about 2 m/s. A main performance goal of the HARPS instrument has therefore been reached.
|The CORALIE spectrometer housed at the Geneva Observatory was used in the discovery of previous exoplanets.|
Credit: The Geneva Extrasolar Planet Search Programmes
This result also demonstrates a gain in efficiency of no less than about 75 times as compared to that achievable with its predecessor CORALIE. That instrument has been operating very successfully at the 1.2-m Swiss Leonard Euler telescope at La Silla and has discovered several exoplanets during the past years. In practice, this means that this new planet searcher at La Silla can now investigate many more stars in a given observing time and consequently with much increased probability for success.
Pedestrian Speeds Achieved with Non-Pedestrian Stability
HARPS is a unique fiber-fed "echelle" spectrograph able to record at once the visible range of a stellar spectrum (wavelengths from 380 – 690 nm) with very high spectral resolving power (better than R = 100,000). Any light losses inside the instrument caused by reflections of the starlight in the various optical components (mirrors and gratings), have been minimized and HARPS therefore works efficiently.
The goal of measuring velocities of stars with an accuracy comparable to that of a pedestrian has required new techniques for the design and construction of this instrument. HARPS is the most stable spectrograph ever built for astronomical applications. A crucial measure in this respect is the location of the HARPS spectrograph in a climatized room in the telescope building. The starlight captured by the 3.6-m telescope is guided to the instrument through an optical fiber from the telescope’s Cassegrain focus.
The spectrograph is placed inside a vacuum tank to reduce to a minimum any movement of the sensitive optical elements because of changes in pressure and temperature. The temperature of the critical components of HARPS itself is kept stable, with less than 0.005 degree variation and the spectrum therefore drifts by less than 2 m/s per night. This is a very small value – 1 m/s corresponds to a displacement of the stellar spectrum on the CCD detector by about 1/1000 the size of one CCD pixel, which is equivalent to 15 nm or only about 150 silicon atoms. This drift is continuously measured by means of a Thorium spectrum which is simultaneously recorded on the detector with an accuracy of only 20 cm/s.
In the case of more conventional instruments, drifts of several hundreds of m/s may occur during one observing night due to the variation of atmospheric pressure (at the rate of about 90 m/s for 1 milli-bar variation) or the ambient air temperature (300 m/s for 1° temperature [Centigrade] variation). The measured nightly drift of HARPS is so small and so smooth that it is possible to compute an average value of this drift with an accuracy of only a few cm/s.
|Our Milky Way galaxy is packed with 400 billion stars and perhaps even more planets.|
During this first commissioning period in February 2003, all instrument functions were tested, as well as the complete data flow system for hardware and software. Already during the second test night, the data-reduction pipeline was used to obtain the extracted and wavelength-calibrated spectra in an entirely automatic way. The first spectra obtained with HARPS will now allow the construction of templates needed to compute the radial velocities of different types of stars.
The second commissioning period in June will then be used to optimize performance of the new instrument. Beginning October 1, 2003, astronomers in the ESO community will have the opportunity to observe with HARPS.
This superb radial velocity machine will also play an important role for the study of stellar interiors by asteroseismology–sometimes called the ‘sounds of a star’ because of the probitive value of detecting its acoustic oscillations. Oscillation modes were recently discovered in the nearby solar-type star Alpha Centauri A from precise radial velocity measurements carried out with CORALIE . HARPS is able to carry out similar measurements on fainter stars, thus reaching a wider range of masses, spectral characteristics and ages.
The HARPS consortium has been granted 100 observing nights per year during a 5-year period at the ESO 3.6-m telescope to perform what promises to be the most ambitious systematic, ground-based search for exoplanets so far implemented worldwide.
During the next 15 years or so, American and European scientists hope to launch more than half a dozen missions to search our corner of the Milky Way galaxy for terrestrial planets.
|A planet exerts a small gravitational pull on its parent star, causing the star to wobble. The motion amplitude depends on the orbital distance and mass of the planet.|
Credit: The Geneva Extrasolar Planet Search Programme
To search for Earth-like planets around stars beyond our solar system, the Kepler Mission , scheduled for launch in 2006, will use a space-borne telescope. Kepler will simultaneously observe 100,000 stars in our galactic "neighborhood," looking for Earth-sized or larger planets within the "habitable zone" around each star – the not-too-hot, not-too-cold zone where liquid water might exist on a planet. One NASA estimate says Kepler should discover 50 terrestrial planets if most of those found are about Earth’s size, 185 planets if most are 30 percent larger than Earth and 640 if most are 2.2 times Earth’s size. In addition, Kepler is expected to find almost 900 giant planets close to their stars and about 30 giants orbiting at Jupiter-like distances from their parent stars. A key criterion for such suitable planets would be whether they reside in habitable zones, or regions sometimes protected by gas giants but with temperate climates and liquid water.
After Kepler, NASA is considering a 2009 launch for SIM, the Space Interferometry Mission . SIM’s primary mission will be to measure distances to stars with 100 times greater precision than now is possible. This will improve estimates of the size of the universe and help astronomers determine the true brightness of stars, and thus learn more about their chemical composition and evolution. SIM also will look for Earth-sized planets in the habitable zones around some 200 stars. SIM will be an interferometer, which means it will combine interacting light waves from its three component telescopes. This interaction, called interference, makes the individual telescopes, which are separated from each other on the spacecraft, act as though they were a single, larger telescope with greater light-gathering ability.
Future missions, such as ESA’s Herschel mission will search for many more and take detailed pictures of stars that might harbor dusty remnants of entire solar systems. As these images become available, astronomers will be able to predict the sizes and orbits of giant planets within the distant solar system.
HARPS has been designed and built by an international consortium of research institutes, led by the Observatoire de Genève (Switzerland) and including Observatoire de Haute-Provence (France), Physikalisches Institut der Universität Bern (Switzerland), the Service d’Aeronomie (CNRS, France), as well as ESO La Silla and ESO Garching. The project team is directed by Michel Mayor (Principal Investigator), Didier Queloz (Mission Scientist), Francesco Pepe (Project Managers Consortium) and Gero Rupprecht (ESO representative).
Related Web Pages
European Southern Observatory
Astrobiology Magazine New Planets
Extrasolar Planets Encyclopedia
Planet Quest (JPL)
Space Interferometry Mission