FRID 21/8/00 4:04 pm Page 2 r bulletin 103 — august 2000 Darwin – The Infrared Space Interferometry Mission C.V.M. Fridlund Space Science Department, ESA Directorate of Scientific Programmes, ESTEC, Noordwijk, The Netherlands Introduction To address fully the recommendations of the ESA already identified interferometry in space Horizon 2000+ Survey Committee for Darwin, it as an important topic in its original Horizon is necessary to: 2000 plan. Consequently, a number of – directly detect exoplanets conferences/workshops were held during the – define and observe the fundamental require- 1980s in order to define the scientific case, and ments for life as we know it even then the search for terrestrial exoplanets – define and observe signposts for the existence figured prominently. When an external survey of life as we know it. committee made its recommendations for the extension of the Horizon 2000 Programme Below we describe the ‘model mission’, (Horizon 2000+), in response to technology resulting from the system-level scientific and developments worldwide, they identified inter- industrial study, that can fulfil these ferometry from space as a potential Cornerstone requirements. The version of Darwin that finally candidate for the new programme. flies may depart significantly from this model mission, in the light of possible collaborations Darwin is a suggested ESA Cornerstone mission, with the express (see below). We also outline the intended purpose of achieving unprecedented spatial resolution in the infrared technology-development schedule, the wavelength region, leading to new astrophysics discoveries and the necessary pre-cursor programmes, and a carrying out of the first direct search for terrestrial exoplanets. The possible framework for collaboration with other detection and study of the latter promises to usher in a new era in space agencies. science and will have an impact on a broad spectrum of science and technology. Ground-based observations of exoplanets The last five years have seen the detection of many planets beyond our Solar System. Since Within the interferometric context, three topics the first detection of a planet around 51 Peg, were identified for further study: reported by Queloz and Mayor in 1995, several – Astrometry. groups have added about 30 planets to the – The search for terrestrial exoplanets, including map in our immediate neighbourhood in the characterisation of their properties and Galaxy. The so-called ‘indirect’ technique used atmospheres and the possible detection of provides very little information about the planet biospheres through remote sensing. itself (essentially its minimum mass, because – Astrophysical imaging at a spatial resolution the planetary orbit’s inclination to our line of 2 – 3 orders of magnitude higher than that sight is unknown). The reflex motion in the foreseen with the Next Generation Space parent star with respect to the common centre Telescope (NGST). of mass of the star-planet system caused by the planet’s gravity is observed as a periodic Of these, the first was considered to be Doppler shift in the stellar spectrum over a (relatively) ‘simpler’ to implement and sub- relatively long period of time. The ground- sequently resulted in the GAIA mission based method’s precision therefore depends proposal and study. The Darwin study on the accuracy with which we can measure concerns itself with the two latter concepts, the star’s spectrum and is currently limited to a and during the mission definition phase the of a few ms-1, allowing the detection of planets possibility of addressing both topics – like Jupiter (12 ms-1) or Saturn (3 ms-1). It exoplanets and imaging – with a single mission therefore appears unlikely that this method was given great weight. could be used to infer the presence of Earth- 20 FRID 21/8/00 4:04 pm Page 3 darwin type planets. The lower limit eventually humanities. Being so cross-disciplinary, therefore, expected to be attainable is more plausibly a the mission can also address the question: planet about the size of Uranus, but at significantly smaller distances. “Are we alone in the Universe?” Nevertheless, important results have been and which is one of mankind’s longest standing are being achieved with this method, including quests. Although this question has been the the detection of the first planetary system topic of vigorous philosophical and religious upsilon Andromedae, which contains three debate for centuries, we have finally arrived at planets with minimum masses of 0.77 MJupiter, a point where technology has advanced 2.11 MJupiter and 4.61 MJupiter. The planets so sufficiently to allow it to be properly addressed. far discovered are also found mainly to have short orbital periods – something that has been In order to answer questions like: clearly demonstrated to be a selection effect – How unique is the Earth as a planet? caused by the period of time during which – How unique is life in the Universe? observations have been carried out being relatively short. Consequently, as time passes, we need to observe other stars and directly longer and longer periods are picked up, and determine the existence and characteristics of there are now a number of confirmed planets in any accompanying bodies. This has hitherto orbits with periods of a few years. Theorists, been impossible because of the influence of the however, have problems explaining the star on the attempts to observe any planet formation of the ‘hot Jupiters’ – the massive circling it. planets orbiting very near their parent stars – as well as the high eccentricities possessed by the Definition of an unambiguous signpost for life is majority of the confirmed planets. another matter completely. First we need to define what is life, and then to determine how Another ‘indirect’ ground-based method is to that life affects its environment. Finally, we need obtain astrometric data and thus track a star’s to define observables that can be obtained with path across the sky, measuring the wobble the level of technology foreseen for the mission. introduced by the rotation around the common In the context of Darwin, we have so far centre of mass of the star-planet system. avoided the first of these questions by instead Reports of the detection of planetary specifying the mission requirements based on companions to some nearby stars have been life as we have observed it on Earth. We thus legion during the last century. Both Barnards disregard speculation about life forms based on star and 61 Cyg have several times become the a chemistry different from that found on our central objects of planetary systems, but none own planet. We then try to imagine the of these observations have ever been differences that would occur if life were confirmed. In contrast, Hipparcos data have nonexistant on Earth, and to use information on recently provided upper masses for a number how life disturbs the equilibrium in the Earth’s of planets, including 51 Peg. ESA’s GAIA atmosphere as our criterion. mission promises statistical surveys of massive planets over large distances. The micro- To fully answer the questions raised above, we arcsecond wobbles introduced in the solar need to: proper motion across the sky, as viewed over – Detect planets within the ‘life zone’ (term distances of order 10 pc are, however, too small coined by Frank Drake), i.e. the orbital radii to be detected by this mission. Moreover, if a where water is found in a liquid state sur- system involves more than one planet, there is rounding other stars. In the Darwin study, the a difficulty in characterising the planets (orbital life zone is defined in terms of a black-body period, mass) uniquely and unambiguously. temperature and a range, and does not a priori take into account atmospheric pressure, The scientific case for studying terrestrial etc. This assumes, of course, that life is based exoplanets on the existence of liquid water. The scientific case for Darwin is easy to state, – Determine the planets’ orbital characteristics, but complex to describe. The phrase to detect which means that we need to repeat the and study Earth-type planets and characterise observations several times. them as possible abodes of life, summarises – Observe the spectrum of the planet (Does it the case nicely, but nevertheless does not have an atmosphere?) and determine its project the complete picture. The simple fact is effective temperature, total flux and diameter that Darwin is not only an astronomy mission, (emitting area x albedo). but also contains elements from geophysics – Determine the composition of the atmosphere (including atmospheric physics), biophysics, viz. the presence of water and ozone/oxygen organic chemistry and philosophy and the for an Earth-type planet, mainly inert gases 21 FRID 21/8/00 4:04 pm Page 4 r bulletin 103 — august 2000 Figure 1. Left: for a Mars/Venus-type planet and hydrogen/ scientific specification for an Invitation to Tender Representative spectra (left) methane atmospheres for Jupiter-type planets (ITT) for industry, and arrived at the end of 1997 of the planets Venus, Earth (Fig. 1a) at the following specification: and Mars at the relevant – Major Goal I: to detect Earth-like planets wavelengths. Right: schematic of the The Darwin Science Advisory Group and orbiting nearby stars and to set constraints the industrial study on the possibility of the existence of ‘life as ESA received the Darwin proposal in response we know it’ on these planets. to a Call for Ideas in 1993, and it was among – Major Goal II: to provide imaging in the 5 to the mission concepts selected for a system- 28 micron band, with spatial resolution an level industrial study. It can be briefly described order of magnitude better than that expected as a nulling interferometer comprising five 1 m- of the Next Generation Space Telescope class telescopes, flying at a distance of about (NGST).