ISO: in from the cold
The cosmic cookery book for planets and life
Molecules scattered in the surroundings of the stars, or found in comets and the atmospheres of planets, hoard many secrets. How did the Solar System form? Why and how did life begin on the Earth? May life exist elsewhere? But knowledge of the chemistry of the Universe has been full of gaps till now.
ISO's ability to detect a whole variety of molecules was unprecedented and its results are now filling the blank pages of the cosmic cookery book. For instance, ISO found clear links between stars, comets and the Earth's origin. Comets are remnants of the raw materials that built the Solar System, and in the bright Comet Hale-Bopp ISO found the mineral olivine, one of the main constituents of the Earth's interior. Olivine was also detected by ISO in the dusty disks surrounding young stars, which are thought to be planetary systems in formation.
"We are reconstructing with great detail the history of the Solar System," said Christoffel Waelkens of the University of Leuven in Belgium. "And we are linking it to other systems that are now in a critical evolutionary phase."
What about life, and the carbon chemistry on which it depends? ISO saw complex carbon-rich molecules in many places of the Universe - in interstellar space in our own Galaxy, the Milky Way, and even in other galaxies. One example is the huge bubble of organic matter that Ralf Siebenmorgen of the ISO Data Centre at Villafranca, Spain, found surrounding a young star in the Chamaeleon constellation. It is a kind of shell-like structure never detected before, made mainly of large molecules with hundreds of atoms of carbon and hydrogen. In principle these molecules could provide building blocks for living organisms.
Closer to home, carbon molecules found by ISO in the atmosphere of the planet Saturn included the first detection of benzene. With a simple ring-shaped molecule, benzene is best known on the Earth as a solvent, but it is the parent of a huge range of molecules used by living organisms.
The appearance of the giant planet Jupiter changes completely at different infrared wavelengths, as observed by ISO. The differences give the experts new information about the chemical composition of the atmosphere and how Jupiter's weather works.
Many young stars are ready to build planets
Is the Solar System a unique pearl in the Universe, the only example of planets orbiting a star? Definitely not. While other astronomers detect unseen planets by their effect on the motion of their parent stars, ISO teams see favourable circumstances for planet-making. The left-over gas and dust that swirls around many new-born stars evolves into a so-called protoplanetary disk which glows with infrared light.
Planets can form from the dust grains. When the process is over, only a thin ring of debris remains. Although no telescope has been able so far to image any alien planet, ISO has detected many of these thin debris disks, which astronomers believe are made of small bodies like comets.
The first proven connection between an alien planet and a thin debris disk came when Carsten Dominik, also at Leiden, found a debris disk surrounding a star where a planet had already been detected by other means. The star is Rho Cancri in the Cancer constellation, and a planet about the size of Jupiter is orbiting around it at a very close distance.
"The disks tell us that the system at least made small bodies, so it is likely that it also made planets," said Harm Habing of Leiden University in the Netherlands. "Our statistical study reveals that 50 percent of all young stars have these debris disks. ISO is breaking the ground for projects that will go in search of planets, even Earth-like planets, in the far future."
Nearly 300 young stars have been identified in a census of four star nurseries examined by ISO, like the one illustrated here. Many were previously unknown, because of the low luminosity of the young stars and the obscuring effect of dust in the star-forming regions.
The birth of the Sun was a key event in our cosmic history, and astronomers learn more about it by studying star formation that continues today. Paradoxically, the process begins in extremely cold conditions.
In egg-like objects called pre-stellar cores, ISO astronomers have detected the earliest stages of star formation. The cosmic eggs are hidden within large dust clouds. By the time the dust disperses and the object inside hatches as a plainly visible star, the main event of star formation is over.
Only radio waves and far-infrared rays can escape from the dust clouds to reveal the earlier stages. An international team led by Derek Ward-Thompson of the University of Wales, Cardiff, UK, obtained the first infrared images of several pre-stellar cores, with cold shells of dust at minus 260°C.
A high-speed chain reaction forms new stars
Ever since a bright, massive star was born in the centre of the Trifid Nebula 100 000 years ago, it has spectacularly illuminated the dust and gas in this well-known cloud in the Sagittarius constellation. But much more has happened in the nebula in the intervening years. The energy emitted by the central star changed the physical and chemical conditions in its environment, and has provoked the quick and nearly simultaneous birth of a second generation of massive stars.
Such a chain reaction in star formation has long been postulated, but ISO saw it with unprecedented detail. Infrared light revealed several very young stars 17 to 60 times heavier than the Sun. Such massive stars will mature rapidly and burn fiercely, so they are likely to cause more star formation, continuing the chain reaction in the Trifid Nebula.
"These sources range from dense cores, apparently still inactive, to more evolved sources undergoing violent episodes of mass ejection," explained José Cernicharo of the Instituto de Estructura de la Materia, Madrid, Spain. "We also see all the clear signs of star formation, such as great condensations of very cold dust - at minus 250°C - evolving towards the proto-stars."
An engineering triumph
From November 1995 until May 1998, all through its operational life in orbit, ISO was one of the coolest objects in the universe. The large cryostat on board the satellite, filled before launch with more than 2 000 litres of superfluid helium, kept the telescope and instruments close to the absolute zero of temperature, minus 273°C. This was essential for an infrared space telescope that had to sense the heat of a cool object a billion light-years away. It also had to point at objects in the sky as accurately as tracking a man at a distance of 1 000 kilometres. ISO was a technological challenge for the 35 companies, headed by Aerospatiale, that built the satellite. ISO's four scientific instruments also pushed technology to its limits. The infrared camera ISOCAM, the photo-polarimeter ISOPHOT and the spectrometers SWS and LWS were built by multinational teams with leaders in France, Germany, the Netherlands and the UK, respectively.
The supply of liquid helium needed to keep ISO chilled set a limit to its operating life. Thanks to excellent engineering and a fortuitous combination of circumstances at launch, the helium lasted over 28 months, much longer than the 18 months required, and made possible about 30 percent more observations.
"It was a very demanding and collaborative effort," said Hans Steinz, ESA's Project Manager for ISO. "But it was a total success as evidenced by the enthusiasm of user scientists, and ISO's technology will be used to build still better space telescopes."
"During the original planned lifetime, the giant star-forming regions in the constellations of Taurus and Orion were inaccessible to ISO," commented Martin Kessler, ISO's Project Scientist. "The extended life brought them into view and gave astronomers an eagerly-awaited chance to unlock some of their secrets."
Frenzied star-making in infrared galaxies
The Antennae, 60 million light-years away, are galaxies in collision. The encounter has triggered star formation within dense dust clouds, transparent to ISO's infrared eye. Laurent Vigroux and Felix Mirabel of CEA-Saclay, France, discovered that the most active star-forming regions in the Antennae are visible only in the infrared. They went undetected before ISO.
What struck astronomers, when they first saw the whole infrared sky with the IRAS satellite in 1983, was that the most luminous objects were certain galaxies that looked faint by visible light. They were very dusty, so that starlight was dimmed while intense infrared emission came from cool dust. Needing a name, astronomers called them "ulirgs", meaning ultra-luminous infrared galaxies. But the experts could not agree about their nature - until ISO settled the issue.
The strange beasts are scenes of collision and merger between two galaxies. Some astronomers imagined that the collision created the ulirg by activating a giant black hole. Others thought that the shock of the collision provoked a frenzy of star-making, called a starburst.
Are the ultra-luminous objects hideouts for monsters or nests for new-born stars? After analysing as many as 60 ulirgs with ISO, a team led by Reinhard Genzel and Dieter Lutz of the Max-Planck-Institut für Extraterrestrische Physik at Garching, Germany, favoured the latter option. Penetrating the dust enshrouding the galaxies they found, not evidence for black-hole activity, but characteristic chemical signatures of starbursts.
"This is the first time we can prove that most if not all of the luminosity in the ulirgs comes from star formation," Reinhard Genzel said. "To understand how, and for how long, such vigorous star formation can occur in these galaxies is now one of the most interesting questions in astrophysics".
Amazing rings in the nearby Andromeda Galaxy, M31, are among the surprises from ISO. Instead of the typical starry spiral seen by visible light, ISO shows several rings of very cold dust, at about minus 260°C, invisible to optical telescopes.
"Seen in the far infrared, M31 is a multiple ringed galaxy rather than the classical spiral type found in the optical," noted Martin Haas of the Max-Planck-Institut für Astronomie, Heidelberg, Germany.
A population explosion of stars, long ago
"One of the major ISO results comes from the deep surveys," said Catherine Cesarsky of CEA-Saclay, France. "They show that most of the star formation in the universe has been so far hidden by the dust".
Looking far into space also means looking far back in time. ISO's deep surveys detected the faintest and farthest objects ever seen at infrared wavelengths. The results reveal an early universe much more violent than expected, with three to four times more stars being formed than the rate inferred from observations by visible light. ISO reached the epoch when the Universe was about one third of its present age. It discovered new populations of infrared galaxies producing stars 100 times faster than our own Galaxy, the Milky Way.
"The intense stages of star formation are often associated with merging and interacting galaxies," said Jean-Loup Puget of the Institut d'Astrophysique Spatiale, near Paris. "This process, considered crucial in the evolution of galaxies, can now be traced to the past through infrared observations."
Thus a key aspect of ISO's legacy is the news that infrared space telescopes will help to solve the problem of how galaxies were born and grew up. Here is a major goal for FIRST, the Far Infrared and Submillimetre Telescope due to be launched by ESA in 2007.
ISO opens its gold mine of data. Many more astronomical surprises are still hidden within nearly 30 000 observations obtained by ISO. They are now being made accessible to all scientists via the archive at the ISO Data Centre (IDC) in Villafranca, Spain. Astronomers can request the data easily through the IDC website, and ask for expert support in Villafranca or any of the National Data Centres in Groningen (Netherlands); Garching and Heidelberg (Germany); Orsay-Saclay (France); Rutherford (UK); and Pasadena, California.
A respected role in world astronomy
The newly appointed Director General of the European Southern Observatory, ESO, is Catherine Cesarsky from France. She was the principal investigator responsible for the camera in ISO, ESA's Infrared Space Observatory. Now she will preside over the completion of ESO's Very Large Telescope in Chile.
Cesarsky personifies the new wave in European astronomy, which is multinational and multi-wavelength. To make new discoveries, astronomers need huge optical and radio telescopes on the ground, and clever satellites to observe the many wavelength bands blocked by the Earth's air.
"Fifteen years ago, astronomers in Europe felt like poor relations," Catherine Cesarsky noted. "The USA had a monopoly of major observing facilities, both on the ground and in space. Now that has changed, thanks to Europe-wide efforts. ESO is completing the world's best ground telescope. Meanwhile ESA too has won a respected role in world astronomy, in particular with ISO. But we'll be secure in that role only with continuous innovation. That means new concepts for facilities and instruments on the ground, new space missions like XMM, Integral, FIRST and Planck, and also serious participation in the Next Generation Space Telescope. We can be sure that our American colleagues won't stand still!"
Last updated: 09 March 2000
Water, water everywhere
The Universe has a different face depending on which instrument pictures it. When seen by visible light in the Hubble Space Telescope it displays neither the youngest stars nor those reaching the final stages of their lives. All cold objects and those hidden within opaque clouds of dust will be missed. They are for the eyes of infrared space telescopes only. A new face of the Universe started to show up when the US-Dutch-British IRAS satellite inaugurated infrared space astronomy in 1983. It was, however, just a foretaste of what was to come.
The European Space Agency's Infrared Space Observatory (ISO) was launched in November 1995 and operated until April 1998. It delved much deeper into the dusty and cool Universe, looking for answers to the secrets of how galaxies, stars and planets formed, and where the crucial ingredients for life come from.
The water that we drink and which fills the world's oceans comes from the stars, and ISO saw water throughout the Universe, even in places where it was not thought to be present. These findings help in understanding how life appeared on Earth, and encourage expectations that living things exist elsewhere.
Thanks to ISO, the cosmic history of the water we live by can be traced for the first time, from starry space to the Solar System. During the violent early stages of starbirth, a young star spews out gas at high speed, generating a shock wave that heats and compresses the hydrogen and oxygen present in the environment. This sets the right conditions for water to form. ISO observed the process in the Orion and Sagittarius nebulae, where star nurseries work as huge water-producing factories.
ISO also detected large amounts of water vapour in the higher atmospheres of all the giant planets of the Solar System. This discovery was unexpected, and it implies that the planets have a continuous supply of cosmic water.
"The water comes simply from the interplanetary dust, which in turn is full of scattered grains from comets," said Helmut Feuchtgruber of the Max-Planck-Institut für Extraterrestriche Physik in Garching, Germany.
Especially exciting was the discovery of water in Saturn's largest moon, Titan, where ESA's interplanetary probe Huygens will examine environmental conditions that may be just too cold to bear life. Athena Coustenis of the Paris-Meudon Observatory and Alberto Salama of the ISO Data Centre at Villafranca, Spain, led the international team that found Titan's water.
"Water vapour makes Titan much richer", Athena Coustenis commented. "It will help us to understand the organic chemistry that took place also in the young Earth, since we are seeing a mixture of elaborate organic molecules closely resembling the chemical soup out of which life emerged."