Introduction

The Planck mission

Planck is a mission of the European Space Agency - ESA. The Planck satellite carried instruments provided by two scientific Consortia funded by ESA member states (in particular the lead countries: France and Italy) with contributions from NASA (USA), and telescope reflectors provided in a collaboration between ESA and a scientific Consortium led and funded by Denmark.

Planck was conceived in 1992, in the wake of the release of the results from the Cosmic Background Explorer (COBE) satellite, notably its measurement of the shape of the spectrum of the cosmic microwave background (CMB), and its detection of the spatial anisotropies of the temperature of the CMB. The latter result in particular led to an explosion in the number of ground-based and suborbital experiments dedicated to mapping of the anisotropies, and to proposals for space experiments both in Europe and the USA.

The main objective of Planck was to measure the spatial anisotropies of the temperature of the CMB, with an accuracy set by fundamental astrophysical limits. Its level of performance enabled Planck to extract essentially all the information in the CMB temperature anisotropies. Planck also measured to high accuracy the polarization of the CMB anisotropies, which encodes not only a wealth of cosmological information, but also provides a unique probe of the thermal history of the Universe during the time when the first stars and galaxies formed. In addition, the Planck sky surveys produced a wealth of information on the properties of extragalactic sources and on the dust and gas in our own Galaxy. The scientific objectives of Planck as conceived in 2005 (4 years before launch) were described in detail in the Bluebook.

The development of Planck began with two proposals presented to the European Space Agency (ESA) in May of 1993, for the Cosmic Background Radiation Anisotropy Satellite (COBRAS) and the Satellite for Measurement of Background Anisotropies (SAMBA). Each of these proposed a payload formed by an offset Gregorian telescope focussing light from the sky onto an array of detectors (based on high-electron-mobility transistor [HEMT] low noise amplifiers for COBRAS and very low temperature bolometers for SAMBA) fed by corrugated horns. The two proposals were used by an ESA-led team to design a payload where a single COBRAS-like telescope fed two instruments, a COBRAS-like Low Frequency Instrument (LFI), and a SAMBA-like High Frequency Instrument (HFI), sharing a common focal plane. A period of study of this concept culminated in the selection by ESA in 1996 of the COBRAS/SAMBA satellite (described in the so-called Redbook) into its programme of scientific satellites. At the time of selection the launch of COBRAS/SAMBA was expected to be in 2003. Shortly after the mission was approved, it was renamed in honour of the German scientist Max Planck (1858-1947), winner of the Nobel Prize for Physics in 1918.

Shortly after its selection, the development of Planck was joined with that of ESA's Herschel Space Telescope, based on a number of potential commonalities, the most important of which was that both missions targeted orbits around the second Lagrangian point of the Sun-Earth system and could therefore share a single heavy launcher. In practice the joint development meant that a single ESA engineering team led the development of both satellites by a single industrial prime contractor, leading to the use of many identical hardware and software subsystems in both satellites, and a synergistic sharing of engineering skills and manpower. The industrial prime contractor, Thales Alenia Space France, was competitively selected in early 2001. Thales Alenia Space France was supported by two major subcontractors: Thales Alenia Space Italy for the Service Module of both Planck and Herschel; and EADS Astrium GmbH for the Herschel Payload Module. There were also contributions from many other industrial subcontractors from all ESA member states (the industrial team is described here ).

In early 1999, ESA selected two Consortia of scientific institutes to provide the two Planck instruments that were part of the payload described in the Redbook: the Low Frequency Instrument was developed by a consortium led by N. Mandolesi of the [http://www.iasfbo.inaf.it/en/ Istituto di Astrofisica Spaziale e Fisica Cosmica] (CNR) in Bologna (Italy); and the High Frequency Instrument by a consortium led by J.-L. Puget of the Institut d'Astrophysique Spatiale (CNRS) in Orsay (France).

In early 2000, ESA and the Danish National Space Institute (DNSI) signed a Letter of Agreement for the provision of the two reflectors that are used in the Planck telescope. DNSI led a Consortium of Danish institutes, which together with ESA subcontracted the development of the Planck reflectors to EADS Astrium GmbH (Friedrichshafen, D), now part of the Airbus group, who have manufactured the reflectors using state-of-the-art carbon-fibre technology.

In total, more than 40 European institutes, and some from the USA and Canada joined forces to constitute the Planck Collaboration, and carry out the development, testing, and in-flight operations of these instruments, as well as the ensuing data analysis and initial scientific exploitation.

The Planck Collaboration's institutes and agencies.

The development history of the Planck satellite is summarised here. It culminated with the successful launch of Planck and Herschel on 14 May 2009. After a period dedicated to Commissioning and Performance Verification, Planck started its planned survey observations on 12 August 2009. It carried on observing for a period of about 30 months, around twice the span originally required, and completed five full-sky surveys with both instruments. The Low Frequency Instrument (LFI), which was able to work at higher temperatures than HFI, continued to survey the sky for a large part of 2013, providing even more data to improve the final Planck results. The last command to the Planck satellite was sent on the 23 October 2013, marking the end of operations.

The Planck data products and papers

The Data Products of Planck have been released in four different stages of increasing scope and quality.

• The first set of scientific data, the Early Release Compact Source Catalogue (ERCSC; Planck-Early-VII), was published in January 2011. At the same time, a set of 26 papers related to astrophysical foregrounds was published in a special issue of Astronomy and Astrophysics (Vol. 536, 2011), among which there is an overview paper (Planck-Early-I).
• The second set of data products (sometimes referred to as Planck Release 1 or “PR1,” because it was the first release of cosmologically useful data) was based on data acquired during the so-called nominal mission, i.e. from start of routine operations to 28 November 2010. These products were based on temperature analysis of the whole sky, and were released in March of 2013. The data and associated scientific results are described in a set of 32 papers in another special issue of A&A (Vol. 571, 2014), among which there is another overview (Planck-2013-I).
• The third set of data products (and second set of cosmological data, hence “PR2”) and scientific results released by Planck, was based on the data acquired during the complete Planck mission from 12 August 2009 to 23 October 2013, and hereafter referred to as the “2015 products.” They are based on both temperature and polarization analysis of the entire sky, and were released between February and July 2015. The data and associated scientific results are described in a set of 28 papers published in a third special issue of A&A (Vol. 594, 2016). Again there is an overview paper paper (Planck-2015-A01).
• The fourth set of scientific data (and third set of cosmological data, hence "PR3") was based on the full mission, focussing on inclusion of the polarization data. The data and associated scientific results are described in a series of 11 papers, including a final "Legacy" which also includes an overview of this release (Planck-2020-A1.

In addition to the above listed four groups of data-release-related papers, the Planck Collaboration has published more than 50 “Intermediate” papers containing further astrophysical investigations. These papers are usually based on data products that are either already public or about to become public at the time of publication.

All of the Planck Collaboration papers are listed in and can be downloaded from [1]. At the current time, we encourage people interested in an overview on Planck to start with the latest overview paper (Planck-2020-A1), and follow references to more specific areas of interest.

The Planck Legacy Archive

The Planck Legacy Archive (PLA) contains all public products originating from the Planck mission, and provides an online interface to select and retrieve them. The majority of the scientific data products from Planck have been produced by the LFI and HFI Data Processing Centres on behalf of the Planck Collaboration.

The data products distributed by PLA are classified into the following categories.

• "Timelines" contain time series of data acquired. The types of data provided are:
  • "semi-raw timelines" containing data samples per detector, which have been very minimally processed after retrieval from the satellite;
  • "calibrated timelines" containing data samples per detector after cleaning and calibration.
• "Rings" are timelines that have been binned into individual periods of fixed spin axis pointing. Each ring traces a quasi-large circle on the sky.
• "Maps" are generally all-sky maps in HEALPix format. There are two major types of Planck maps:
  • "frequency maps" are maps as observed at one of the nine frequency channels of Planck, containing at least temperature, and in the later versions also polarization, and existing in many varieties of maps, depending on which detectors and/or time coverage is included in their production;
  • "component maps" are maps of diffuse emission of specific physical components, including the CMB, Galactic, and extragalactic foregrounds, which are constructed from Planck observations using different methods.
• The "Likelihood code" is the software used by the Planck Collaboration to extract the values of cosmological parameters from the Planck data. The code is bundled with the data set (from Planck and other experiments) that it needs to run.
• "Catalogues" contain lists of compact or point-like sources extracted from the Planck maps. The basic Planck catalogues have been extracted without regard to the type of source, but some specialized lists of specific source types are also available.
• "Cosmological data" contain results of some of the cosmological analyses that formed the main objective of Planck, i.e. CMB angular power spectra, cosmological model parameters, etc.

In addition to the above, the PLA also includes a wide variety of additional products:

• external scientific data that were used in the generation of Planck products;
• data characterizing the Planck payload;
• operational data.

All of the Planck products are labelled according to their release: ERCSC-A 2012; PR1 2013; PR2 2015; and PR3 2018. In addition, the product most recommended for use is labelled as the "Legacy" product. The PLA interface allows the user to search the Archive using a wide variety of parameters. It also provides the possibility to extract parts of the products, e.g. a section of the sky, and in some cases to modify them. The PLA also makes it easy to transfer products to generic data analysis tools for further analysis, e.g. Aladin for maps, and Topcat for catalogues.

Finally, the PLA also contains some selected data products that are based on Planck data, but have not been produced by the Planck Collaboration. These data are labelled as "Community" products.

All the PLA products can be accessed via its graphical interface. Some of the products can also be extracted via a machine interface.

This Explanatory Supplement

This Explanatory Supplement (ES) has been built by the Planck Collaboration and the Planck Science Office. It contains:

• general technical information on the Planck satellite, its payload, and its operations;
• specific information for each of the products being distributed by the PLA.

The ES can be accessed online independently or can be called directly from the PLA interface (in this case it will direct the user to the appropriate sections). In general, it will display descriptions appropriate to the "Legacy" products. However, descriptions specific for each of the four Planck releases can be found at the end of each of the ES sections; the background colour of the ES page identifies the release it refers to.

We emphasize that the ES contains a bare minimum of necessary information on the data products. We strongly recommend that users read the scientific papers by the Planck Collaboration that are closest to their own application. Those papers contain the most relevant and useful information on the quality and limitations of the Planck products, which has often not been fully captured in the ES.

For more information

A complete list of Planck publications can be found here.

Suggestions or questions should be sent to the Helpdesk.

Acknowledgments

The development of \Planck\ has been supported by: ESA; CNES and CNRS/INSU-IN2P3-INP (France); ASI, CNR, and INAF (Italy); NASA and DoE (USA); STFC and UKSA (UK); CSIC, MICINN, JA, and RES (Spain); Tekes, AoF, and CSC (Finland); DLR and MPG (Germany); CSA (Canada); DTU Space (Denmark); SER/SSO (Switzerland); RCN (Norway); SFI (Ireland); FCT/MCTES (Portugal); and PRACE (EU). A description of the \Planck\ Collaboration and a list of its members, including the technical or scientific activities in which they have been involved, can be found at this site.