This article is a list of notable unsolved problems in

astronomy Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, g ...

. Some of these unsolved problems in astronomy are theoretical, meaning that existing theories seem incapable of explaining a certain observed phenomenon or experimental result. The others are experimental, meaning that there is a difficulty in creating an experiment to test a proposed theory or investigate a phenomenon in greater detail. Some pertain to one-off events, unusual occurrences that have not repeated and whose causes therefore remain unclear.

Planetary astronomy

Exoplanetary

* How unusual is our

Solar System The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...

? Some observed

planetary systems A planetary system is a set of gravitationally bound non- stellar objects in or out of orbit around a star or star system. Generally speaking, systems with one or more planets constitute a planetary system, although such systems may also cons ...

contain

Super-Earth A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below those of the Solar System's ice giants, Uranus and Neptune, which are 14.5 and 17 times Earth's, respectively. The term "super-Earth" refers only to ...

s and

Hot Jupiter Hot Jupiters (sometimes called hot Saturns) are a class of gas giant exoplanets that are inferred to be physically similar to Jupiter but that have very short orbital periods (). The close proximity to their stars and high surface-atmosphere tem ...

s that orbit very close to their stars (even closer than Mercury). Systems with Jupiter-like planets in Jupiter-like orbits appear to be rare. Is it because of our lack of data, given the difficulty of observing exoplanets? Or can it be explained by the

grand tack hypothesis In planetary astronomy, the grand tack hypothesis proposes that Jupiter formed at 3.5 AU, then migrated inward to 1.5 AU, before reversing course due to capturing Saturn in an orbital resonance, eventually halting near its current orbit at 5.2 AU ...

Solar system

* Orbiting bodies and rotation: ** Are there any non-dwarf

planets beyond Neptune Following the discovery of the planet Neptune in 1846, there was considerable speculation that another planet might exist beyond its orbit. The search began in the mid-19th century and continued at the start of the 20th with Percival Lowell's ...

? ** What is the explanation for the elongated orbits of the extreme trans-Neptunian objects? ** Rotation rate of Saturn: *** Why does the magnetosphere of Saturn rotate at a rate close to that at which the planet's clouds rotate? *** What is the true rotation rate of Saturn's deep interior? * Satellite geomorphology: ** What is the origin of the chain of high mountains that closely follows the equator of Saturn's moon Iapetus? *** Is it the remnant of hot and fast-rotating young Iapetus? *** Is it the result of material (either from the rings of Saturn or its own ring) that over time collected upon the surface?

Stellar astronomy and astrophysics

Solar cycle The solar cycle, also known as the solar magnetic activity cycle, sunspot cycle, or Schwabe cycle, is a nearly periodic 11-year change in the Sun's activity measured in terms of variations in the number of observed sunspots on the Sun's surfa ...

: ** How does the Sun generate its periodically reversing large-scale magnetic field? ** How do other solar-like stars generate their magnetic fields, and what are the similarities and differences between stellar activity cycles and that of the Sun? ** What caused the Maunder Minimum and other grand minima, and how does the solar cycle recover from a minimum state? *

Coronal heating problem A corona ( coronas or coronae) is the outermost layer of a star's atmosphere. It consists of plasma. The Sun's corona lies above the chromosphere and extends millions of kilometres into outer space. It is most easily seen during a total so ...

: ** Why is the Sun's corona (atmosphere layer) so much hotter than the Sun's surface? ** Why is the

magnetic reconnection Magnetic reconnection is a physical process occurring in highly conducting plasmas in which the magnetic topology is rearranged and magnetic energy is converted to kinetic energy, thermal energy, and particle acceleration. Magnetic reconnecti ...

effect many orders of magnitude faster than predicted by standard models? *

Space weather Space weather is a branch of space physics and aeronomy, or heliophysics, concerned with the time varying conditions within the Solar System, including the solar wind, emphasizing the space surrounding the Earth, including conditions in the ...

prediction: ** How does the Sun produce strong southward pointing magnetic fields in solar coronal mass ejections that lead to

geomagnetic storms A geomagnetic storm, also known as a magnetic storm, is a temporary disturbance of the Earth's magnetosphere caused by a solar wind shock wave and/or cloud of magnetic field that interacts with the Earth's magnetic field. The disturbance that d ...

? How can we predict solar and geomagnetic superstorms? * What is the origin of the stellar mass spectrum? That is, why do astronomers observe the same distribution of stellar masses – the

initial mass function In astronomy, the initial mass function (IMF) is an empirical function that describes the initial distribution of masses for a population of stars. The IMF is an output of the process of star formation. The IMF is often given as a probability d ...

– apparently regardless of the initial conditions? * Supernova: What is the exact mechanism by which an implosion of a dying star becomes an explosion? *

p-nuclei p-nuclei (''p'' stands for proton-rich) are certain proton-rich, naturally occurring isotopes of some elements between selenium and mercury inclusive which cannot be produced in either the s- or the r-process. Definition The classical, gro ...

: What astrophysical process is responsible for the nucleogenesis of these rare isotopes? *

Fast radio burst In radio astronomy, a fast radio burst (FRB) is a transient radio pulse of length ranging from a fraction of a millisecond to 3 seconds, caused by some high-energy astrophysical process not yet understood. Astronomers estimate the average FRB rel ...

s (FRBs): What causes these transient radio pulses from distant galaxies, lasting only a few milliseconds each? Why do some FRBs repeat at unpredictable intervals, but most do not? Dozens of models have been proposed, but none has been widely accepted. * The

Oh-My-God particle The Oh-My-God particle was an ultra-high-energy cosmic ray detected on 15 October 1991 by the Fly's Eye camera in Dugway Proving Ground, Utah, U.S. It is the highest-energy cosmic ray ever observed. This particle's energy was unexpected an ...

and other

ultra-high-energy cosmic ray In astroparticle physics, an ultra-high-energy cosmic ray (UHECR) is a cosmic ray with an energy greater than 1 EeV (1018 electronvolts, approximately 0.16 joules), far beyond both the rest mass and energies typical of other cosmic ray parti ...

s: What physical processes create cosmic rays whose energy exceeds the GZK cuttoff? * Nature of

KIC 8462852 Tabby's Star (also known as Boyajian's Star and WTF Star, and designated KIC 8462852 in the Kepler Input Catalog) is an F-type main-sequence star in the constellation Cygnus (constellation), Cygnus approximately from Earth. Unusual ligh ...

, commonly known as Tabby's Star: What is the origin of unusual luminosity changes of this star?

Galactic astronomy and astrophysics

Galaxy rotation problem The rotation curve of a disc galaxy (also called a velocity curve) is a plot of the orbital speeds of visible stars or gas in that galaxy versus their radial distance from that galaxy's centre. It is typically rendered graphically as a plot, ...

: Is

dark matter Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not a ...

responsible for differences in observed and theoretical speed of stars revolving around the centre of galaxies, or is it something else? * Age–metallicity relation in the Galactic disk: Is there a universal age–metallicity relation (AMR) in the Galactic disk (both "thin" and "thick" parts of the disk)? Although in the local (primarily thin) disk of the

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...

there is no evidence of a strong AMR, a sample of 229 nearby "thick" disk stars has been used to investigate the existence of an age–metallicity relation in the Galactic thick disk, and indicate that there is an age–metallicity relation present in the thick disk. Stellar ages from asteroseismology confirm the lack of any strong age-metallicity relation in the Galactic disc. *

Ultraluminous X-ray source An ultraluminous X-ray source (ULX) is an astronomical source of X-rays that is less luminous than an active galactic nucleus but is more consistently luminous than any known stellar process (over 1039 erg/s, or 1032 watts), assuming that it radiat ...

s (ULXs): What powers X-ray sources that are not associated with

active galactic nuclei An active galactic nucleus (AGN) is a compact region at the center of a galaxy that has a much-higher-than-normal luminosity over at least some portion of the electromagnetic spectrum with characteristics indicating that the luminosity is not prod ...

but exceed the Eddington limit of a

neutron star A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Except for black holes and some hypothetical objects (e.g. w ...

or stellar black hole? Are they due to

intermediate mass black hole An intermediate-mass black hole (IMBH) is a class of black hole with mass in the range 102–105 solar masses: significantly more than stellar black holes but less than the 105–109 solar mass supermassive black holes. Several IMBH candidate obje ...

s? Some ULXs are periodic, suggesting non-isotropic emission from a neutron star. Does this apply to all ULXs? How could such a system form and remain stable? * What is the origin of the Galactic Center GeV excess? Is it due to the annihilation of dark matter particles or a new population of millisecond pulsars? * The infrared/TeV crisis: Lack of attenuation of very energetic gamma-rays from extragalactic sources.

Black holes

Gravitational singularities A gravitational singularity, spacetime singularity or simply singularity is a condition in which gravity is so intense that spacetime itself breaks down catastrophically. As such, a singularity is by definition no longer part of the regular sp ...

: Does

general relativity General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics ...

break down in the interior of a black hole due to

quantum effects Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, ...

torsion Torsion may refer to: Science * Torsion (mechanics), the twisting of an object due to an applied torque * Torsion of spacetime, the field used in Einstein–Cartan theory and ** Alternatives to general relativity * Torsion angle, in chemistry Bi ...

, or other phenomena? *

No-hair theorem The no-hair theorem states that all stationary black hole solutions of the Einstein–Maxwell equations of gravitation and electromagnetism in general relativity can be completely characterized by only three independent ''externally'' observabl ...

: ** Do black holes have an internal structure? *** If so, how might the internal structure be probed? *

Supermassive black hole A supermassive black hole (SMBH or sometimes SBH) is the largest type of black hole, with its mass being on the order of hundreds of thousands, or millions to billions of times the mass of the Sun (). Black holes are a class of astronomical ob ...

s: ** What is the origin of the

M–sigma relation The M–sigma (or ''M''–''σ'') relation is an empirical correlation between the stellar velocity dispersion ''σ'' of a galaxy bulge and the mass M of the supermassive black hole at its center. The ''M''–''σ'' relation was first presented ...

between supermassive black hole mass and galaxy velocity dispersion? * The formation of high-redshift quasars: ** How do the most distant quasars grow their supermassive black holes up to 10¹⁰ solar masses so early in the history of the universe (with redshift > 6-7)? *

Black hole information paradox The black hole information paradox is a puzzle that appears when the predictions of quantum mechanics and general relativity are combined. The theory of general relativity predicts the existence of black holes that are regions of spacetime from wh ...

and

black hole radiation Hawking radiation is theoretical black body radiation that is theorized to be released outside a black hole's event horizon because of relativistic quantum effects. It is named after the physicist Stephen Hawking, who developed a theoretical ar ...

: ** Do black holes produce thermal radiation, as expected on theoretical grounds? *** If so, and black holes can evaporate away, what happens to the information stored in them (since quantum mechanics does not provide for the destruction of information)? Or does the radiation stop at some point leaving black hole remnants? * Firewalls: Does a firewall exist around a black hole?Originally published
in Quanta, December 21, 2012. * Final parsec problem: Supermassive black holes appear to have merged, and what appears to be a pair in this intermediate range has been observed, in

PKS 1302-102 PKS 1302-102 is a quasar in the Virgo constellation, located at a distance of approximately 1.1 Gpc (around 3.5 billion light-years). It has an apparent magnitude of about 14.9 mag in the V band with a redshift of 0.2784. The quasar is hosted ...

. However, theory predicts that when supermassive black holes reach a separation of about one parsec, it would take billions of years to orbit closely enough to merge – more than the age of the universe. *

Naked singularity In general relativity, a naked singularity is a hypothetical gravitational singularity without an event horizon. In a black hole, the singularity is completely enclosed by a boundary known as the event horizon, inside which the curvature of space ...

: Is

cosmic censorship hypothesis The weak and the strong cosmic censorship hypotheses are two mathematical conjectures about the structure of gravitational singularities arising in general relativity. Singularities that arise in the solutions of Einstein's equations are typically ...

correct? Does a

naked singularity In general relativity, a naked singularity is a hypothetical gravitational singularity without an event horizon. In a black hole, the singularity is completely enclosed by a boundary known as the event horizon, inside which the curvature of space ...

exist?

Cosmology

Cosmological principle In modern physical cosmology, the cosmological principle is the notion that the spatial distribution of matter in the universe is homogeneous and isotropic when viewed on a large enough scale, since the forces are expected to act uniformly throu ...

: ** Is the universe homogeneous and isotropic at large enough scales, as claimed by the

cosmological principle In modern physical cosmology, the cosmological principle is the notion that the spatial distribution of matter in the universe is homogeneous and isotropic when viewed on a large enough scale, since the forces are expected to act uniformly throu ...

and assumed by all models that use the

Friedmann–Lemaître–Robertson–Walker metric The Friedmann–Lemaître–Robertson–Walker (FLRW; ) metric is a metric based on the exact solution of Einstein's field equations of general relativity; it describes a homogeneous, isotropic, expanding (or otherwise, contracting) universe tha ...

, including the current version of the ΛCDM model, or is the universe

inhomogeneous Homogeneity and heterogeneity are concepts often used in the sciences and statistics relating to the uniformity of a substance or organism. A material or image that is homogeneous is uniform in composition or character (i.e. color, shape, siz ...

or anisotropic? ** Is the CMB dipole purely kinematic, or does it signal anisotropy of the universe, resulting in the breakdown of the FLRW metric and the cosmological principle? ** Is the

Hubble tension Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. In other words, the farther they are, the faster they are moving ...

evidence that the cosmological principle is false? ** Even if the cosmological principle is correct, is the

the right metric to use for our universe? ** Are the observations usually interpreted as the

accelerating expansion of the universe Observations show that the expansion of the universe is accelerating, such that the velocity at which a distant galaxy recedes from the observer is continuously increasing with time. The accelerated expansion of the universe was discovered duri ...

rightly interpreted, or are they instead evidence that the

is false? ** Copernican principle: Are cosmological observations made from Earth representative of observations from the average position in the universe? *

Dark matter Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not a ...

: ** What is the identity of dark matter? ** Is dark matter a

particle In the physical sciences, a particle (or corpuscule in older texts) is a small localized object which can be described by several physical or chemical properties, such as volume, density, or mass. They vary greatly in size or quantity, from ...

? If so, is it a

WIMP Wimp, WIMP, or Wimps may refer to: Science and technology * Weakly interacting massive particle, a hypothetical particle of dark matter * WIMP (computing), the "window, icon, menu, pointer" paradigm * WIMP (software bundle), the web stack of Wind ...

, an axion, the lightest

superpartner In particle physics, a superpartner (also sparticle) is a class of hypothetical elementary particles predicted by supersymmetry, which, among other applications, is one of the well-studied ways to extend the standard model of high-energy physics. ...

(LSP), or some other particle? ** Do the phenomena attributed to dark matter point not to some form of matter but actually to an extension of gravity? *

Dark energy In physical cosmology and astronomy, dark energy is an unknown form of energy that affects the universe on the largest scales. The first observational evidence for its existence came from measurements of supernovas, which showed that the univ ...

: ** What is the cause of the observed

(the de Sitter phase)? ** Are the observations rightly interpreted as the

, or are they evidence that the

is false? ** Why is the energy density of the dark energy component of the same magnitude as the density of matter at present when the two evolve quite differently over time? *** Could it be simply that we are observing at exactly the

right time ''Right Time'' is the 1976 studio album debut of influential reggae band the Mighty Diamonds. The album, released by Virgin Records after they signed the Mighty Diamonds following a search for talent in Jamaica, is critically regarded as a regg ...

? ** Is dark energy a pure cosmological constant or are models of quintessence such as

phantom energy Phantom energy is a hypothetical form of dark energy satisfying the equation of state with w < -1. It possesses negative kinetic energy, and predic ...

applicable? ** Do early dark energy models resolve the

? *

Baryon asymmetry In physical cosmology, the baryon asymmetry problem, also known as the matter asymmetry problem or the matter–antimatter asymmetry problem, is the observed imbalance in baryonic matter (the type of matter experienced in everyday life) and antib ...

: Why is there far more

matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic part ...

than

antimatter In modern physics, antimatter is defined as matter composed of the antiparticles (or "partners") of the corresponding particles in "ordinary" matter. Antimatter occurs in natural processes like cosmic ray collisions and some types of radioac ...

in the

observable universe The observable universe is a ball-shaped region of the universe comprising all matter that can be observed from Earth or its space-based telescopes and exploratory probes at the present time, because the electromagnetic radiation from these ob ...

? *

Cosmological constant problem In cosmology, the cosmological constant problem or vacuum catastrophe is the disagreement between the observed values of vacuum energy density (the small value of the cosmological constant) and theoretical large value of zero-point energy sugges ...

: ** Why does the

zero-point energy Zero-point energy (ZPE) is the lowest possible energy that a quantum mechanical system may have. Unlike in classical mechanics, quantum systems constantly fluctuate in their lowest energy state as described by the Heisenberg uncertainty pri ...

of the

vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often di ...

not cause a large

cosmological constant In cosmology, the cosmological constant (usually denoted by the Greek capital letter lambda: ), alternatively called Einstein's cosmological constant, is the constant coefficient of a term that Albert Einstein temporarily added to his field eq ...

? *** What cancels it out? *

Size Size in general is the magnitude or dimensions of a thing. More specifically, ''geometrical size'' (or ''spatial size'') can refer to linear dimensions ( length, width, height, diameter, perimeter), area, or volume. Size can also be m ...

and shape of the universe: ** The diameter of the

is about 93 billion light-years, but what is the size of the whole universe? Is it infinite? ** What is the 3- manifold of comoving space, i.e. of a comoving spatial section of the universe, informally called the "shape" of the universe? *** Neither the curvature nor the topology is presently known, though the curvature is known to be "close" to zero on observable scales. The cosmic inflation hypothesis suggests that the shape of the universe may be unmeasurable, but, since 2003, Jean-Pierre Luminet, et al., and other groups have suggested that the shape of the universe may be the Poincaré dodecahedral space. Is the shape unmeasurable; the Poincaré space; or another 3-manifold? * Cosmic inflation: ** Is the theory of cosmic inflation in the very early universe correct? *** And, if so, what are the details of this epoch? ** What is the hypothetical scalar field that gave rise to this cosmic inflation? ** If inflation happened at one point, is it self-sustaining through inflation of quantum-mechanical fluctuations, and thus ongoing in some extremely distant place? *

Horizon problem The horizon problem (also known as the homogeneity problem) is a cosmological fine-tuning problem within the Big Bang model of the universe. It arises due to the difficulty in explaining the observed homogeneity of causally disconnected region ...

: ** Why is the distant universe so homogeneous when the

Big Bang theory The Big Bang event is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models of the Big Bang explain the evolution of the observable universe from the ...

seems to predict larger measurable

anisotropies Anisotropy () is the property of a material which allows it to change or assume different properties in different directions, as opposed to isotropy. It can be defined as a difference, when measured along different axes, in a material's physic ...

of the night sky than those observed? *** Cosmological

inflation In economics, inflation is an increase in the general price level of goods and services in an economy. When the general price level rises, each unit of currency buys fewer goods and services; consequently, inflation corresponds to a reduct ...

is generally accepted as the solution, but are other possible explanations such as a variable speed of light more appropriate? *

: If ΛCDM is correct, why are measurements of the Hubble constant failing to converge? *

Axis of evil The phrase "axis of evil" was first used by U.S. President George W. Bush and originally referred to Iran, Iraq, and North Korea. It was used in Bush's State of the Union address on January 29, 2002, less than five months after the 9/11 attac ...

: Some large features of the microwave sky at distances of over 13 billion light years appear to be aligned with both the motion and orientation of the solar system. Is this due to systematic errors in processing, contamination of results by local effects, or an unexplained violation of the Copernican principle? * Why is there something rather than nothing?, origin and future of the universe: ** How did the conditions for anything to exist arise? ** Is there potentially an infinite amount of unknown astronomical phenomena throughout our entire universe? ** Is the universe heading towards a

Big Freeze The heat death of the universe (also known as the Big Chill or Big Freeze) is a hypothesis on the ultimate fate of the universe, which suggests the universe will evolve to a state of no thermodynamic free energy, and will therefore be unabl ...

, a

Big Rip In physical cosmology, the Big Rip is a hypothetical cosmological model concerning the ultimate fate of the universe, in which the matter of the universe, from stars and galaxies to atoms and subatomic particles, and even spacetime itself, is ...

, a Big Crunch, or a Big Bounce, or is it part of an infinitely recurring

cyclic model A cyclic model (or oscillating model) is any of several cosmological models in which the universe follows infinite, or indefinite, self-sustaining cycles. For example, the oscillating universe theory briefly considered by Albert Einstein in 1930 t ...

? * Multiverse: ** Is there a multiverse? Are there an infinite amount of universes in the multiverse? How to test the hypothesis? Is the multiverse one of an infinite amount of multiverses? Are these infinite multiverses and all the infinite universes cyclical? ** Should we embrace the anthropic principle to explain unsolved scientific puzzles such as the

cosmological constant problem In cosmology, the cosmological constant problem or vacuum catastrophe is the disagreement between the observed values of vacuum energy density (the small value of the cosmological constant) and theoretical large value of zero-point energy sugges ...

Extraterrestrial life

* Is there other life in the Universe? Especially: ** Is there other intelligent life? ** Is there potentially an infinite amount of extraterrestrial genera throughout our universe? *** If so, what is the explanation for the

Fermi paradox The Fermi paradox is the discrepancy between the lack of conclusive evidence of advanced extraterrestrial life and the apparently high a priori likelihood of its existence, and by extension of obtaining such evidence. As a 2015 article put it, ...

? * Nature of

Wow! signal The Wow! signal was a strong narrowband radio signal detected on August 15, 1977, by Ohio State University's Big Ear radio telescope in the United States, then used to support the search for extraterrestrial intelligence. The signal appeared ...

: ** Was this singular event a real signal? *** If so, what was its origin?

References

{{DEFAULTSORT:Unsolved problems in astronomy Astronomy-related lists

Astronomy Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, g ...