Planck (spacecraft)

Planck was designed to measure tiny temperature and polarization variations in the cosmic microwave background across the entire sky. ESA developed the mission with contributions from European institutions and NASA, and launched it with the Herschel Space Observatory on 14 May 2009 aboard an Ariane 5 from Kourou.¹

The spacecraft operated near the second Sun–Earth Lagrange point, where thermal and radio conditions favored stable observations. A passive cooling system and a chain of increasingly cold stages brought the detectors close to absolute zero. Its Low Frequency Instrument used radiometers, while the High Frequency Instrument used bolometers; together they covered approximately 30 to 857 gigahertz. The broad frequency range was essential because Galactic dust, synchrotron radiation, free–free emission, and other foregrounds can mimic or obscure the primordial signal.

Planck scanned the sky repeatedly as the spacecraft rotated and its spin axis followed the Sun. Repeated coverage allowed researchers to separate persistent celestial signals from instrumental noise and to construct maps of temperature, polarization, and several forms of secondary emission.

The mission’s products extended beyond a single CMB image. Public releases included calibrated frequency maps, component-separated CMB maps, catalogs of compact sources, and maps of Galactic dust and polarized emission. Planck also detected thousands of clusters of galaxies through the Sunyaev–Zeldovich effect, in which hot cluster gas distorts CMB photons. Its measurements of diffuse Galactic emission became valuable to studies of star formation, magnetic fields, and the interstellar medium, not merely to precision cosmology.²

Planck’s CMB measurements strongly supported a spatially close-to-flat universe dominated by dark energy and cold dark matter, while refining estimates of the universe’s age, composition, and expansion history. The 2018 final release inferred an age of roughly 13.8 billion years within the standard six-parameter cosmological model.

The mission provided an especially precise measurement of the angular scale of acoustic peaks—features produced by sound waves in the early photon–baryon plasma. These peaks constrain the baryon density, matter density, and primordial fluctuation spectrum. Planck also found that the primordial fluctuations are highly close to scale-invariant and nearly Gaussian, results consistent with broad classes of inflationary models but not a proof of any single inflation theory. Differences between Planck’s inferred expansion rate and some later, local measurements contributed to the continuing Hubble-tension debate.

Planck’s most consequential technical challenge was not only detecting the CMB but characterizing the instrument itself. Detector noise, beam shape, gain variations, and far sidelobes had to be modeled alongside bright foregrounds; polarization analysis was particularly sensitive to systematic errors and to polarized dust in the Milky Way.

The spacecraft’s cryogenic resources were consumed gradually, so the two instruments ended science operations at different times: the High Frequency Instrument stopped in January 2012 after its coolant was exhausted, while the Low Frequency Instrument continued until October 2013. Planck’s maps also revealed previously catalogued phenomena in new detail, including the cosmic infrared background produced by unresolved dusty galaxies and the gravitational-lensing imprint of intervening large-scale structure.² After operations ended, the spacecraft was moved to a distant heliocentric disposal orbit rather than left near the Lagrange point.

Cosmic microwave background

Relic electromagnetic radiation from the early universe, now observed as a nearly uniform microwave background.

Lagrange point

A position in a two-body system where gravitational and orbital effects can allow a spacecraft to maintain a useful relative configuration.

Bolometer

A highly sensitive detector that measures absorbed radiation through its effect on temperature or electrical properties.

Sunyaev–Zeldovich effect

The spectral distortion of CMB photons caused by their scattering from energetic electrons in hot galaxy-cluster gas.

Cosmic infrared background

The accumulated infrared emission from distant, mostly unresolved galaxies, particularly those containing obscured star formation.