ESA Science & Technology - INFO 18-1996: Value of Hipparcos Catalogue shown by planet assessments

The first detailed findings from Hipparcos, recently published in Astronomy and Astrophysics Letters, confirm the existence of planets around other stars. Hipparcos astronomers plucked out their data on three stars suspected of possessing attendant planets. Their distances, measured far more accurately than ever before, enables the astronomers to rule out, in two cases, the possibility that the supposed planets might be small stars. The discovery of alien planets in the first astronomical step towards fashioning a proper science out of the speculations about life beyond the solar system. Hipparcos makes a decisive contribution by setting an upper limit to their masses.

Astronomers at the Geneva Observatory caused a sensation last year when they reported slight motions in the star 51 Pegasi, due to a massive planet orbiting around it. With a ground-based telescope they detected small shifts in the wavelength of light as 51 Pegasi moved slowly under the influence of its invisible companion. This year, astronomers at San Francisco State University confirmed the discovery and have subsequently reported two similar cases, in the stars 47 Ursae Majoris and 70 Virginis. Uncertainties about the orientation of the planets' orbits and the distances of the stars left a wide margin of doubt about the masses of the candidate planets.

Accurate rangefinding by Hipparcos puts the star 47 Ursae Majoris at a distance of 46 light-years. Calculations then set an upper limit on the mass of the companion at 7 to 22 times the mass of Jupiter, the Sun's largest planet. The Sun itself is a thousand times more massive that Jupiter, and theorists believe that the smallest true star would have a mass of 80 Jupiters. Below that mass, the object cannot burn hydrogen in the nuclear fashion, which is the most characteristic source of energy for stars.

In the range between 17 and 80 Jupiter masses an object is called a brown dwarf. It can in theory derive a little energy by burning heavy hydrogen, or deuterium. Even the "worst-case" mass quoted here for the companion of 47 Ursae Majoris, 22 Jupiter masses, is only a maximum, not a measurement. So the companion is almost certainly a true planet with less than 17 times the mass of Jupiter.

For the star 70 Virginis, the distance newly established by Hipparcos is 59 light-years. Even on the least favourable assumptions about its orbit, the companion cannot have more than 65 Jupiter masses. It could be brown dwarf rather than a planet, but not a true star. Much more ambiguous is the result for 51 Pegasi. Its distance is 50 light-years and theoretically the companion could have more than 500 Jupiter masses, or half the mass of the Sun. This is a peculiar case anyway, because the companion is very close to 51 Pegasi.

Small planets of the size of the Earth might be more promising as abodes of life than the large planets detectable by present astronomical methods. Space scientists are now reviewing methods of detecting the presence of life on alien planets by detecting the infrared signature of ozone in a planet's atmosphere. Ozone is a by-product of oxygen gas, which in turn is supposed to be generated only by life similar to that on the Earth. Meanwhile the detection of planets of whatever size is a tour de force for astronomers, and by analogy with the Solar System one may suppose that large planets are often likely to be accompanied by smaller ones.

"Hipparcos was not conceived to look for planets," comments Michael Perryman, ESA's project scientist for Hipparcos, "and this example of assistance to our fellow-astronomers involves a very small sample of our measurements. But it is a timely result when we are considering planet-hunting missions for the 21st Century. The possibilities include a super-Hipparcos that could detect directly the wobbles in nearby stars due to the presence of planets." Hipparcos Catalogue ready for use The result from Hipparcos on alien planets coincides with the completion of the Hipparcos Catalogue and the distribution of the data to collaborating scientists. The Catalogue lists the positions, distances, motions and brightnesses of 118 000 stars, within a self-consistent framework for the whole sky. The results include remarkable detail on the orbits of double stars and the changing light output of variable stars. Thanks to the unprecedented accuracy of space observations with a specially conceived satellite, the Hipparcos Catalogue gives positions of the stars to much better than a millionth of a degree. This is over one hundred times more accurate than the most careful fixes of stars from observatories on the ground.

With the Catalogue's completion, the Hipparcos mission achieves its purpose of revolutionizing astrometry, the positional science that has underpinned mankind's studies of the Universe since the satellite's namesake Hipparchus the Greek surveyed the sky in the 2nd Century BC. Another pioneer of astrometry was Tycho the Dane (16th Century AD) and he is commemorated in a second product of the Hipparcos mission. The Tycho Catalogue, which is nearing completion, lists a far larger number of stars, slightly more than a million. Their positions will be less precise than in the Hipparcos Catalogue, but still far better than the ground-based results, and will include valuable data on the colours of stars.

The first chance to cull discoveries from the Hipparcos data resides with the teams in nine European countries who have laboured to achieve this breakthrough in astronomy. For some team members the effort goes back thirty years, to the first proposal of an astrometric satellite in France in 1966. ESA approved the mission in 1980 and Hipparcos was launched in August 1989. Despite finding itself in the wrong orbit, Hipparcos operated successfully until March 1993. Three more years have gone into data analysis, using computers in a dozen institutes across Europe for the most elaborate calculations in the history of astronomy. Members of the teams will now have privileged use of the data for nearly a year, before the release of the catalogues to the world-wide astronomical community in April 1997.

The new precision in star-fixing promises an extraordinary harvest of new results, on subjects ranging from asteroids to cosmology. A vivid picture of the stars in motion in our corner of the Milky Way Galaxy is one expected outcome. Hipparcos has more than doubled the number of known variable stars, and has discovered many thousands of new double or multiple star systems.

Striding the light-years by parallax The study of stars with candidate planets is a dramatic example of Hipparcos's new determinations of the distances of stars by the parallax principle. Many other discoveries will flow from it.

Parallax is an unfamiliar name for a familiar concept, akin to stereoscopic vision. People judge distances in nearby scenes from the difference in direction of the two eyes when focused on an object. Military rangefinders use the same principle, with more widely separated optics. Astronomers adapt the Earth's orbit to make a huge rangefinder.

At opposite seasons, the Earth is on opposite sides of the Sun, at vantage points 300 million kilometres apart. As a result, the bearings of stars change a little. Nearby stars shift more than very distant stars, and astronomers can measure their distances by trigonometry. Until now, unavoidable inaccuracies in observing the directions of stars from the ground have meant that the distances of only the closest stars can be measured directly by parallax, out to about 100 light-years. Even at short ranges the margin of uncertainty is often wide.

Long chains of inference and observation extend the distance scale to much farther objects, including galaxies millions or billions of light-years away. From the resulting estimates, astronomers try to calculate the age of the Universe and arrive at conclusions about its origin and evolution. The foundation for these reckonings has been decidedly shaky.

By measuring seasonal changes of direction to an accuracy of better than a millionth of degree, the Hipparcos mission has taken a great stride across the light-years and transformed the art of determining astronomical distances. Hipparcos has pushed the practical range of direct and accurate distance measurement out to about 1000 light-years. That means rangefinding for a thousand times as many stars as before. As for the nearer stars, parallax measurements of their distances are now far more accurate, thanks to Hipparcos.

The star 70 Virginis, which figures in the planets study, illustrates the overwhelming power of the new data. Ground-based measurements of the shift in its position were wildly discrepant. The largest reported distance, 102 light-years, was more than three times the smallest, 29 light-years. By contrast, the uncertainty in the actual distance of 59.1 light-years, measured by Hipparcos, has been whittled down to about one per cent, or roughly half a light-year.