The Planck mission will collect and characterise radiation from the Cosmic Microwave Background (CMB) using sensitive radio receivers operating at extremely low temperatures. These receivers will determine the black body equivalent temperature of the background radiation and will be capable of distinguishing temperature variations of about one microkelvin. These measurements will be used to produce the best ever maps of anisotropies in the CMB radiation field.

The Spacecraft

The Planck spacecraft is 4.2 metres high and has a maximum diameter of 4.2 metres, with a launch mass of around 1.9 tonnes. The spacecraft comprises a service module, which houses systems for power generation and conditioning, attitude control, data handling and communications, together with the warm parts of the scientific instruments, and a payload module. The payload module consists of the telescope, the optical bench, with the parts of the instruments that need to be cooled - the sensitive detector units - and the cooling systems.

The Planck spacecraft is made up of two major components, the payload module and the service module.

The payload module comprises:

The telescope, equipped with primary and secondary mirrors that collect microwave radiation and direct it onto the instrument focal plane units and a baffling system to restrict the entry of stray radiation into the telescope optics The cryogenic focal plane units of the two instruments The instrument cooling chains (excluding the compressors and control electronics)

The service module is an octagonally shaped bus and contains the systems needed to operate the spacecraft:

Power: generation, storage, conditioning and distribution of electric power Attitude and Orbit Control: measurement of the spacecraft's attitude using star trackers and sun sensors, and changing the spacecraft's attitude or orbit by means of hydrazine thrusters.

Control and Data Management: receipt, storage and execution of ground commands, autonomous operationo send back data and receive commands

Planck Capabilities

Planck will provide a map of the Cosmic Microwave Background (CMB) field at all angular resolutions greater than 10 arcminutes and with a temperature resolution of the order of one part in 106. The simultaneous mapping of the sky at a wide range of frequencies will enable the separation of the Galactic and extragalactic foreground radiation from the primordial cosmological background signal.

The Questions Planck Will Answer

The questions that Planck will seek answers to include:

  • What are the (more precise) values of fundamental cosmological parameters such as the Hubble constant?
  • Can it be shown conclusively that the early Universe passed through an inflationary phase?
  • What is the nature of the dark matter that dominates the present Universe?

Mission Lifetime

Planck has a nominal operational lifetime of fifteen months from the end of the Calibration and Performance Verification Phase.

Launch date: 14-May-2009 13:12 UT
Mission end: end 2011 (extended mission, HFI & LFI); mid-2013 (extended mission, LFI only)
Launch vehicle: Ariane 5 ECA
Launch mass: about 1900 kg
Mission phase: Routine operations
Orbit: Lissajous orbit about the second Lagrange point of the Earth-Sun system (L2), with an average amplitude of about 400 000 km.
Objectives: The primary science goals of Planck include:
  • Mapping the Cosmic Microwave Background anisotropies with improved sensitivity and angular resolution
  • Testing inflationary models of the early Universe
  • Measuring the amplitude of structures in the Cosmic Microwave Background
  • Determination of Hubble constant
  • Perform measurements of Sunyaev-Zel'dovich effect



Planck's Predecessors

The Cosmic Background Explorer (COBE) was launched on 18 November 1989. COBE determined that the CMB exhibits anisotropies at a level of one part in 105 and showed that the CMB spectrum matched that of a black body with a temperature of 2.725 K ± 2 mK.

The Wilkinson Microwave Anisotropy Probe (WMAP) was launched on 30 June 2001 and has made measurements of the CMB enabling the creation of a map of the anisotropies with much higher spatial and temperature resolution and improved accuracy compared to the COBE results.

In 1983 the US-Dutch-British IRAS satellite inaugurated infrared space astronomy by mapping 250 000 cosmic infrared sources and large areas of extended emission.


The Planck scientific instrument complement comprises two instruments, LFI, a radio receiver array covering the lower frequency range, and HFI, a bolometric detector array covering the higher frequencies. The instruments share a common telescope. Principal Investigator (PI) consortia provide the instruments and telescope.

Low Frequency Instrument

The Low Frequency Instrument (LFI) is designed to produce high-sensitivity, multi-frequency measurements of the microwave sky in the frequency range 27 to 77 GHz (wavelength range 11.1 to 3.9 mm).

LFI Performance Goals

Centre frequency (GHz)




Bandwidth (GHz)




Beamwidth (arcminutes, FWHM)




Detector technology

High electron mobility transistor (HEMT) radio receiver arrays

Detector temperature (K)

~ 20

Cooling technology

H2 sorption cooler

Average ΔT/T1 per pixel*




Average ΔT/T2 per pixel*




Flux sensitivity per pixel* (mJy)




High Frequency Instrument

The High Frequency Instrument (HFI) is designed to produce high-sensitivity, multi-frequency measurements of the diffuse sky radiation in the frequency range 84 GHz to 1 THz (wavelength range 3.6 to 0.3 mm).

HFI Performance Goals

Centre frequency (GHz)







Bandwidth (GHz)







Beamwidth (arcminutes, FWHM)







Detector technology

Spider bolometer arrays,
neutron transmutation doped (NTD) germanium thermistors

temperature (K)

~ 0.1

Cooling technology

H2 sorption cooler + Joule-Thomson cooler + 3He/4He dilution cooler

Average ΔT/T3
per pixel*







Average ΔT/T4
per pixel*




Flux sensitivity
per pixel* (mJy)







* A pixel is a square whose side is the full width, half maximum (FWHM) extent of the beam. These sensitivity figures are calculated for the average integration time per pixel. The integration time will be very inhomogeneously distributed over the sky and will be much higher in certain regions.

3Sensitivity (1 σ) to intensity (Stokes I) fluctuations, measured in thermodynamic temperature units × 10-6, relative to the average temperature of the CMB (2.73 K), achievable after two sky surveys (14 months).

4Sensitivity (1 σ) to polarised intensity (Stokes U and Q) fluctuations, measured in thermodynamic temperature units × 10-6, relative to the average temperature of the CMB (2.73 K), achievable after two sky surveys (14 months).

Measurement Results

The measurements made by the two instruments will be combined and used to produce a full-sky map of the anisotropies in the Cosmic Microwave Background (CMB) with unprecedented precision. This map will in turn be used to derive a wealth of cosmological information, including an accurate determination of the values of the main parameters that characterise the large-scale structure and evolution of the Universe.


The telescope design is an off-axis tilted Gregorian system, offering the advantages of no blocking of the optical path combined with compactness. The eccentricity and tilt angle of the secondary mirror and the off-axis angle obey the Dragone-Mizuguchi condition, which allows the system to operate without significant degradation over a large focal plane array, while simultaneously minimizing the polarization effects introduced by the telescope.

The baffling system is composed of two elements. The shield element is a large, self-supporting and roughly conical structure covered with multi-layer insulation (MLI), which surrounds the telescope and focal plane instruments. Together with the optical bench, it defines the optical enclosure. It has two important functions, reducing the level of straylight (which at the chosen orbit is in large part due to the spacecraft itself) and promoting the radiative cooling of the optical enclosure towards deep space. The baffle element consists of one half of a conically shaped surface that links the focal plane instruments to the bottom edge of the sub-reflector. The function of the baffle is to shield the detectors from thermal radiation originating within the optical enclosure.

Telescope characteristics

Off-axis tilted Gregorian

Primary mirror

1.9 × 1.5 m, off-axis paraboloid

Secondary mirror

1.1 × 1.0 m, off-axis paraboloid


Planck will help provide answers to some of the most important questions in modern science: how did the Universe begin, how did it evolve to the state we observe today, and how will it continue to evolve in the future? Planck's objective is to analyse, with the highest accuracy ever achieved, the remnants of the radiation that filled the Universe immediately after the Big Bang - this we observe today as the Cosmic Microwave Background.

Mission Name

Planck, originally named COBRAS/SAMBA, was renamed on approval of the mission in 1996 in honour of the German scientist Max Planck (1858-1947) who won the Nobel Prize for Physics in 1918. J.C. Mather and G.F. Smoot have received the Nobel Prize for Physics in 2006 for their discovery of the blackbody nature of the Cosmic Microwave Background radiation and the small-scale deviations from the blackbody curve.

Mission Objectives

  • Perform measurements of Cosmic Microwave Background anisotropies
  • Test inflationary models of the early Universe
  • Measure amplitude of structures in the Cosmic Microwave Background
  • Perform measurements of Sunyaev-Zeldovich effect


Frequency Instrument
Description 83 GHz - 1 THz
Array of 52 bolometric detectors, operated at 0.1K
Principal Investigator Jean-Loup Puget,
Institut d'Astrophysique Spatiale (Orsay, France)
Deputy Principal Investigator François Bouchet,
Institut d'Astrophysique de Paris (Paris, France)
Frequency Instrument
Description 27 - 77 GHz
Array of 22 tuned radio receivers, operated at 20K
Principal Investigator Nazzareno Mandolesi,
Istituto di Tecnologie e Studio delle Radiazioni Extraterrestri (Bologna, Italy)
Deputy Principal Investigator Marco Bersanelli,
Universita' degli Studi di Milano (Milan, Italy)
Orbital Insertion

Planck was carried into space on 14 May 2009, at 13:12:02 UTC, by an Ariane 5 ECA launcher, from the Guiana Space Centre, Kourou, French Guiana. Planck was launched together with ESA's Herschel spacecraft.

Within 30 minutes after launch, and about two minutes from each other, the two spacecraft were released and each placed on their individual escape trajectory toward L2, the second Lagrange point of the Sun-Earth system. Upon separation, Planck was spin stabilised at 1 rpm.

About six weeks after launch, following a set of trajectory control manoeuvres, Planck will reach its operational orbit: a Lissajous orbit with an average amplitude of about 400 000 km around the L2 point at a distance of around 1.5 million km from Earth in the anti-Sun direction.

The commissioning of the spacecraft is performed during the journey to L2 and is expected to last until the spacecraft reaches its operational orbit.

Location of L2 (not to scale)

At L2, Planck will perform a major manoeuvre to enter its Lissajous orbit about the Lagrange point, with the Sun-spacecraft-Earth angle limited to 15°. Lissajous orbits are the natural motion of a satellite around a collinear libration point in a two-body system and require less momentum change to be expended for station keeping than halo orbits, where the satellite follows a simple circular or elliptical path about the libration point.

Orbits about L2 are dynamically unstable; small departures from equilibrium grow exponentially with a time constant of about 23 days. Planck will use its propulsion system to perform orbit maintenance manoeuvres.

Choice of Orbit

The Planck spacecraft is spin stabilised at 1 rpm and normally operates with its spin axis pointing directly away from the Sun. The line-of-sight of the telescope is positioned at an angle of 85° to the spin axis and the instruments scan a circular sector of the celestial sphere with a radius of 85° once per spacecraft revolution. In order to view the celestial poles, the spin axis can be moved up to 10° away from the anti-Sun direction.

The anti-Sun pointing strategy reduces the effects of solar radiation to a minimum. However, the Earth and Moon can also be intense sources of both straylight and thermal radiation, and reducing their effects drives the choice of orbit. Near Earth orbits are eliminated mainly because the large thermal flux renders it extremely difficult to reach low temperatures at the focal plane of the telescope or to achieve the required thermal stability. The nearest possible far-Earth orbits are those around one of the Lagrangian points of the Earth-Moon system. These orbits, which share the lunar motion around the Earth, are rendered unsuitable by the fact that either the Earth or the Moon is often not very far from the telescope line-of-sight. Simulations indicate that if this type of orbit were chosen, at least 35% of the acquired data would have to be discarded due to poor thermal or straylight conditions, leading not only to lower sky coverage but also to a less efficient removal of systematic effects.

The optimum orbit, selected as a result of a trade-off among the various payload requirements, several spacecraft technical constraints (most importantly those related to communication with the ground station), and the transfer-to-orbit cost, is a Lissajous orbit around the L2 Lagrangian point of the Earth-Sun system. At this location the Sun, the Earth, and the Moon are all located behind the payload, where their undesirable effects are at the lowest possible level, both in terms of location and of flux. In addition, this is the only orbit in which the antennas, which provide Earth communication for the spacecraft, are also continuously pointed away from the payload, thereby minimizing the potential effects of RF interference.

As Planck orbits L2, it makes one rotation about the Sun per year. The spacecraft spin axis has to be rotated at the same rate in order to remain Sun pointed. This is achieved by making regular manoeuvres that will be combined with periodically moving the spin axis out of the ecliptic plane to obtain full sky coverage. In addition to keeping the spin axis pointed within 10° of the anti-Sun direction to keep the payload in shadow, it must also be kept within 15° of the Earth direction in order to keep the Earth in the field-of-view of the communications antenna.


The Planck science payload consists of two instruments that are designed to study the Cosmic Microwave Background (CMB) radiation field by making high sensitivity measurements in the frequency range 27 GHz to 1 THz, and a telescope that collects the microwave radiation and focuses it onto the instrument detector arrays.

Planck's primary science objectives are to:

  • Map Cosmic Microwave Background anisotropies
  • Test inflationary models of the early universe
  • Measure the amplitude of structures in the Cosmic Microwave Background
  • Perform measurements of the Sunyaev-Zeldovich effect

Planck has the ability to:

  • Detect much smaller temperature variations in the CMB than previous missions
  • Perform CMB measurements with a higher angular resolution than ever before
  • Measure over a wider band of frequencies to enhance the separation of the CMB from interfering foreground signals

The CMB anisotropy map produced using Plank's observations will be markedly superior to those currently available and will be used to set constraints on the values of the main parameters that govern the large scale structure of the Universe.

The Cosmic Microwave Background (CMB) preserves a picture of the Universe as it was about 380 000 years after the Big Bang, and can reveal the initial conditions for the evolution of the Universe. Planck’s main objective is to measure the fluctuations of the CMB with an accuracy set by fundamental astrophysical limits. The spacecraft will chart the most accurate maps yet of the CMB.

Planck's instrument detectors are so sensitive that temperature variations of a few millionths of a degree will be distinguishable. This unrivalled sensitivity together with the large and smooth surface of its telescope and its unprecedented wavelength coverage make Planck the most sophisticated 'time machine' ever.

Additionally, many objects of great interest to astronomers are concealed within or behind clouds of gas and dust. In the early stages of their formation, stars and planets are surrounded by the gas and dust clouds from which they are being created. Galactic cores and most of the remnants of the early Universe are also hidden from view by dust clouds. The dust particles in these clouds are comparable in size to the wavelength of visible light and are therefore efficient at scattering or absorbing radiation at these wavelengths. Infrared radiation is less affected by these clouds - the longer the wavelength, the thicker the dust cloud that it can penetrate.

Planck's major objectives are:

To determine the large-scale properties of the Universe with high precision. Planck will take a census of the main constituents of the Universe and build a history of their evolution in time. For example, it will accurately determine the density of normal matter, allowing us to calculate the total number of atoms in the visible Universe.

It will also investigate the nature and determine the amount of dark matter — a strange substance that does not emit or reflect electromagnetic radiation, but whose presence can be inferred from its gravitational pull on normal detectable matter, and which may account for around 90% of matter in today's Universe. Planck will also investigate the nature of dark energy, a form of energy that is theorised to account for the Universe's expansion at an accelerating rate.

To test theories of inflation, a period of extremely rapid expansion that gave birth to the Universe and that is the current explanation for some of its observed fundamental features. Planck's measurements will make it possible to study how and why such rapid expansion may have been triggered, how it evolved, and its consequences on our still-expanding Universe.

To search for primordial gravitational waves. These waves are expected to have been present at the time when inflation took place. Gravitational waves distort the fabric of space-time and carry information about the mechanism and the energies at which they were generated. Should Planck succeed in detecting these signatures, it would provide strong evidence for inflation.

To search for 'defects' in space, that would indicate that the Universe harboured local inhomogeneities in its very early phases. For instance, the presence of cosmic strings would hint at exotic physical phenomena that may have contributed to the origin and evolution of the structures that we see in the Universe today.

To study the origin of the structures we see in the Universe today. Planck's accurate measurements of the variations in the microwave background provide a direct probe into the initial inhomogeneities that slowly grew into the largest structures that we see today: galaxies, clusters of galaxies, and ubiquitous large voids. Comparing the structure of the Universe then and now allows testing the very complex theories of structure formation. In addition, the photons of the microwave background can tell us the time and manner of formation of the first stars of our Universe.

To study our and other galaxies in the microwave.Planck will study the Milky Way and map, for the first time, the large- scale distribution of cold dust along the spiral arms. It will also be the first to map, in detail and in 3D, the magnetic field which permeates the Milky Way. Beyond our own Galaxy, Planck will observe distant radio and dusty galaxies and investigate how they form stars. At larger scales, it will study near and distant clusters of galaxies and extract clues on how they formed and evolved.

ESA's Planck satellite was launched on 14 May 2009 and operated for over four years, scanning the whole sky several times at microwave and sub-millimetre frequencies. Its main goal was to take the most detailed photograph ever of the tiny fluctuations present in the Cosmic Microwave Background (CMB), the most ancient light that has travelled across the Universe.

The first image of the entire sky showing these minute differences and based on the data collected during Planck's first 15.5 months of observations, was released in March 2013. Planck has provided the most accurate snapshot made of the matter distribution in the early Universe, only 380 000 years after the Big Bang. Fluctuations in the CMB correspond to the cosmic seeds that would evolve into all the structure observed in the Universe today – from stars and planets to galaxies and galaxy clusters.

Planck's precise data enables cosmologists to investigate a huge variety of models for the origin and evolution of the cosmos. The new image of the CMB has confirmed that the standard model of cosmology is a very good description of the Universe. Dominated by the as yet unexplained dark matter and dark energy, the cosmos we live in appears to have begun almost 14 billion years ago with an early period of accelerated expansion, called inflation, during which the seeds of cosmic structure were embedded in the Universe.

The data from Planck have allowed cosmologists to set very tight constraints on many parameters of the standard model, including the Hubble constant (H0), which describes the expansion rate of the Universe today, the densities of baryonic matter, dark matter and dark energy (Ωb, Ωm, ΩΛ), and the spectral index (ns), which describes the relative amount of primordial fluctuations – the seeds of nascent cosmic structures – on different scales.

But hidden in the detail provided by Planck, there was also a hint of something more fundamental beneath the surface: a number of anomalies in the data do not perfectly agree with the predictions of the standard model. Some of these anomalies were found for the first time in the Planck data, while there had been evidence of others in previous experiments. Theoreticians continue to speculate about the implications of these anomalies and to investigate possible ways of extending the standard model.

Another piece of the puzzle about the origin of the cosmos is now being analysed: a small fraction of the CMB is polarised, and the instruments on board Planck were sensitive to this signal. Polarisation carries additional information about the very early phases of the Universe's history and particularly about inflation itself. Results from the analysis of the CMB polarisation, will help cosmologists to constrain the origin and evolution of the cosmos with ever greater precision.

Beyond its main goal – imaging the Cosmic Microwave Background (CMB) in unprecedented detail – ESA's Planck satellite has also probed the build-up of structure in the Universe in several different ways.

Since the CMB is the most ancient light that has travelled across the Universe, its photons have encountered and interacted with a multitude of structures that were taking shape during their almost 14-billion year journey. Stars and galaxies started to form a few hundred million years after the Big Bang and, over time, increasingly large objects were assembled, with galaxy clusters arising in the densest knots of the cosmic web that permeates the Universe.

One of the interactions between CMB photons and cosmic structure, leaving a lasting mark on the CMB, is gravitational lensing, the bending of light caused by massive objects. The dark matter halos in which galaxies and galaxy clusters are embedded act as lenses and deflect the path of photons. Analogous to what happens when light rays pass through a glass lens, the effect of gravity on the photons causes distortion to the image of distant sources.

The CMB photons are deflected multiple times as they cross the large-scale distribution of cosmic structure, resulting in tiny distortions to the already mottled pattern of the CMB temperature fluctuations. This gravitational lensing effect on the CMB, discovered with previous experiments on small patches of the sky, was mapped over the entire sky for the first time using data from Planck. Cosmologists have used this measurement to reconstruct an all-sky map of the gravitational potential that distorts the CMB, studying in ever greater detail the evolution of structure formation in the Universe.

Another type of interaction also takes place when the CMB photons encounter individual structures – galaxy clusters - in the large-scale distribution of cosmic matter. Galaxy clusters are the largest objects in the Universe that are bound by gravity: besides galaxies, they contain large amounts of ionised gas and even larger amounts of dark matter.

When photons from the CMB scatter off free electrons in the gas that permeates galaxy clusters, their energy is changed in a distinctive way. This is known as the Sunyaev-Zel'dovich effect and allows scientists to identify galaxy clusters from observations of the CMB. Planck has found over a thousand galaxy clusters across the entire sky, including almost 400 brand new detections. Among these are also several superclusters and the first observation of inter-cluster gas bridging two clusters.

With its broad spectral coverage, Planck is also sensitive to another type of background radiation that peaks at shorter wavelengths than the CMB, the Cosmic Infrared Background (CIB). In contrast to the CMB, which is the diffuse light from the early Universe, the CIB is a cumulative background with contributions from all of the star-forming galaxies across cosmic history.

Observations of fluctuations in the CIB performed with Planck at different wavelengths have been used to trace the large-scale distribution of star-forming galaxies at different epochs in cosmic history. Besides the CIB, which is a cumulative radiation, Planck also detected the brightest galaxies as individual, discrete sources. Once removed and catalogued, what had been considered a foreground nuisance for cosmologists has turned into a valuable resource for studying the formation and evolution of galaxies – in particular dusty, star-forming galaxies and those hosting an active galactic nucleus.

The main goal of ESA's Planck satellite was to map the Cosmic Microwave Background (CMB), the most ancient light that has travelled across the Universe. But before scientists in the Planck Collaboration could start exploiting this pool of cosmological information long and meticulous work was required to remove several foreground signals hiding the CMB. The strongest source of foreground is our Galaxy, and the information contained in these layers is helping astronomers to paint a new view of the Milky Way.

The contaminants include emissions from galaxies, galaxy clusters and, most crucially, from diffuse material within our own Galaxy, the Milky Way. The careful removal of foreground emissions that is necessary to uncover the cosmological potential of Planck's data had a very valuable by-product: the delivery of large data sets to many other fields in astrophysics.

The interstellar medium (ISM) that permeates the Milky Way shines brightly in all of Planck's wavelength channels via a variety of radiation processes. The main causes of this diffuse radiation are free electrons at the longest wavelengths, and dust grains at the shortest wavelengths probed by Planck.

Data from Planck were used to compile maps of the various components of the ISM, revealing in the process some new players contributing to our Galaxy's emission.

With the new data, astronomers studied a mysterious emission that was detected several years ago at microwave wavelengths, confirming that its origin can be explained by spinning dust grains and that it is widespread in the ISM. Another study of Planck's maps enabled the astronomers to better quantify the amount of gas in molecular clouds where stars are being born. Locating and quantifying gas in molecular clouds is a vexed issue in the study of star formation since molecular hydrogen, the gas from which stars take shape, does not emit and must be traced indirectly. To this aim, astronomers have exploited the data from Planck to obtain the first all-sky map of the emission from carbon monoxide, a molecule that traces the densest pockets of gas where star formation is taking place across the Galaxy.

A study using some of the early data collected by Planck produced a chart of the densest and coldest cores across the Galaxy, where the earliest steps of star formation take place. This catalogue proved a very helpful resource for follow-up studies of these stellar cradles with higher resolution observations, including some performed with ESA's Herschel Space Observatory.

Mass About 1900 kg at launch
Dimensions 4.2 m high, 4.2 m maximum diameter
Launcher Ariane 5 ECA from Guiana Space Centre
Mission Lifetime 15 months nominal from end of Calibration and Performance Verification Phase
Wavelength Microwave: 27 GHz to 1 Thz
Telescope 1.9×1.5m primary mirror (1.5m projected aperture)

The Planck spacecraft is made up of two major components, the payload module and the service module.

The payload module comprises:

  • The telescope, equipped with primary and secondary mirrors that collect microwave radiation and direct it onto the instrument focal plane units and a baffling system to restrict the entry of stray radiation into the telescope optics
  • The cryogenic focal plane units of the two instruments
  • The instrument cooling chains (excluding the compressors and control electronics)

The service module is an octagonally shaped bus and contains the systems needed to operate the spacecraft:

  • Power: generation, storage, conditioning and distribution of electric power
  • Attitude and Orbit Control: measurement of the spacecraft's attitude using star trackers and sun sensors, and changing the spacecraft's attitude or orbit by means of hydrazine thrusters
  • Control and Data Management: receipt, storage and execution of ground commands, autonomous operation of the spacecraft in the absence of a ground station link, storage and management of observation and housekeeping data
  • RF Communications: linking the spacecraft with the ground station to send back data and receive commands

The service module also houses those parts of the instruments that do not require cooling, and the compressors and control electronics for the instrument cooling chains.

Other News
Other News
Planck Upholds Standard Cosmology

The Planck team has finally released its full-mission data, revealing a remarkably detailed view of our universe and our galaxy. Scientists presented the results at a Planck conference in Ferrara, Italy, in December, but the official analysis papers are only now coming out. Most were posted on the Planck Publications website on February 5th, with a few stragglers still in the wings.

Planck launched in 2009 to study the cosmic microwave background (CMB), the relic radiation from the universe’s birth. Density fluctuations in the universe’s earliest moment spawned the splotchy pattern we see in the CMB and, in turn, served as seeds for the growth of cosmic structure. Understanding why the CMB looks the way it does therefore helps us understand the entire universe.

Observing in nine frequencies spanning 30 to 857 GHz, Planck mapped the CMB’s temperature and (in seven frequencies) polarization, with angular resolutions between 33 and 5 arcminutes, depending on the frequency. It shut down on schedule four years later in 2013.

The team released the temperature observations from the mission’s first 15 months in 2013. These data were mostly in agreement with the predictions of the standard cosmological model. Since then, the team has been working to analyze the full, four-year data set.

How Astronomers Find the Universe in the CMB

The strength of temperature variations (vertical) is plotted against their angular sizes (horizontal, approximate). The red line is the standard cosmological model, the blue dots are Planck data. Credit: Planck Collaboration

This endeavor is a challenging one, explains Planck team member Charles Lawrence (JPL). Cosmologists start with the splotchy CMB pattern. From that they calculate what’s called the power spectrum, which reveals the strength of the CMB’s fluctuations at different angular scales. (The power spectrum is the wiggly graph at right.) The power spectrum is the cornerstone of the whole effort: it’s this statistical map that cosmologists base their CMB analysis on.

The cosmologists then make some assumptions about what kind of universe they’re dealing with — in astrospeak, they assume the standard lambda-CDM model, which includes (1) a particular solution to the general relativistic equations of gravity, (2) a universe that looks basically the same on large scales and is expanding, (3) an early period of stupendous expansion called inflation, and (4) quantum fluctuations that seeded today’s large-scale matter distribution.

From there, they start tweaking the assumptions, like a dressmaker tucking and letting out a dress pattern until it fits right. They could even chuck any assumption that proves to be bad. Eventually, they find the pattern that most successfully fits the CMB.

The amazing thing is, this method works. It works really well. That’s because back when the universe cooled down enough to become transparent to the CMB’s radiation (about 380,000 after the Big Bang), the universe was simple. By simple, I mean the universe was basically a hot, bland soup of particles and dark matter and there weren’t any chemical reactions going on. So scientists can actually figure out, to very high precision, the exact setup that would create the CMB we observe.

Planck’s Cosmology Results

This composite map of the Milky Way Galaxy from the Planck mission combines several types of emission: synchrotron (from charged particles corkscrewing in magnetic fields), free-free (from electrons scattering off ions), spinning dust, and warm dust. Planck scientists must subtract out all galactic emission in order to see the cosmic microwave background. Credit: Planck Collaboration

The 2015 release upholds that of 2013, with only slight tweaks to various cosmological parameters. It still overwhelmingly favors an early universe defined entirely by six parameters, no matter how many ways the team pushed and prodded the data. These parameters are:

The density of baryonic matter in the first few minutes of the universe.

The density of cold dark matter at that same time How far sound waves had traveled when the CMB photons were released — also known as the “sound horizon” or the size of baryon acoustic oscillations.

The fraction of CMB photons over the universe’s history that have scattered off particles set free by radiation from stars/quasars ionizing the neutral hydrogen filling the cosmos.

The strength of the initial density fluctuations on a physical scale of about 65 million light-years (20 megaparsecs) at the end of inflation.

How the strength of the density fluctuations on various scales at the end of inflation changes with scale.

From these, the team can calculate just about anything you please, such as the universe’s age and its expansion rate. The exact values depend on which data subsets you want to include, but here are some notable ones from the team’s overview paper:

Age of universe: 13.799 +/- 0.038 billion years: that means we know the age of the universe to within 38 million years. Hubble parameter: 67.8 +/- 0.9 km/s/megaparsec (this is the universe’s rate of expansion.

Fraction of universe’s content that is “dark energy”: 69.2 +/- 1.2%.


The latest Planck data say some interesting things about the universe. For one, the universe’s expansion rate, called the Hubble parameter, is still lower than what astronomers previously calculated using supernovae (about 73 km/s/Mpc). That was a surprise in the 2013 release, and it’s still odd. Several other measurements have also been pushing the Hubble constant down, so it looks like a lower expansion rate is here to stay. Maybe there’s some new physical ingredient at work, but we don’t know yet.

In the discussion of what dark matter is, one idea is that it’s its own antiparticle and so if two of its particles collide, they’ll go poof. There’s no sign of dark matter annihilation in the physics needed to explain the CMB observations, although Planck does leave the door open for the level of annihilation suggested as an explanation for diffuse gamma-ray emission from the Milky Way’s center. And it looks like there are definitely only 3 flavors of neutrino.

There’s still the strange problem of the missing galaxy clusters. The Planck team finds a certain lumpiness in the CMB, which should match up with the lumps in the distribution of matter in the universe (a.k.a. cosmic structure, which is made up of galaxy clusters). But Planck predicts about 2.5 times more clusters than are actually observed. This could be due to error in the estimates from either side, or due to new physics.

One result is that the era of reionization — basically, when the universe’s galaxies really started lighting up with stars — is later than estimated using data from Planck’s predecessor, WMAP. WMAP had favored reionization at a redshift of 10 (470 million years after the Big Bang), but Planck pegs it at 8.8 (560 million years after the Big Bang).

“For many cosmologists, I would say that it is a relief,” says David Spergel (Princeton), who worked on the WMAP team. Scientists studying early star formation had a hard time explaining the earlier start time from WMAP, so a slightly later start is a good thing.

Then there are the implications for inflation. No. 6 in the list of parameters (how the strength of the density fluctuations changes with angular scale), is called ns, or the scalar spectral index. It’s important because it describes the state of affairs at the end of inflation, and the fluctuations it measures are the ones that started sound waves sloshing in the universe’s primordial plasma and ultimately led to the CMB we see. Planck finds a value of 0.968, which means that the strength of the fluctuations is slightly larger on larger scales — predicted by most inflation models. This offset has a slight effect on galaxies’ formation rate over time.

The Planck team also did its own analysis of how big any gravitational waves triggered by inflation would be in their data, an analysis separate from the joint analysis done with the BICEP2/Keck Array folks. Adding together the Planck-only result and the Planck-BICEP2 result, the team found an upper limit on the ratio of gravitational waves’ strength to the density fluctuations’ strength of 0.08, slightly lower than the one from the joint analysis (0.12) and the Planck 2013 analysis (0.11). (The new Planck-only upper limit is 0.10.)

These results home in on some of the simpler types of inflation. These involve an inflation spawned by the decay of a single energy field, a field that decreased slowly compared to the universe’s expansion rate. (Given that the observable universe expanded at least 5 billion trillion times in 10 nano-nano-nano-nanoseconds, that’s not that slow.) The energy scale implied for inflation is less than 2 x 1016 gigaelectron volts, on par with the level expected for the merger of the strong, weak, and electromagnetic forces into one (called the Grand Unified Theory). Physicists think these forces were united in the first mini-moment of the universe, then broke apart. Their breakup might somehow be connected to inflation.