List of unsolved problems in astronomy

Some of the 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 unresolved questions in astronomy pertain to one-off events, unusual occurrences that have not repeated and whose causes therefore remain unclear.

Planetary astronomy

• Planetary systems: How does accretion form planetary systems?^[1] Where did Earth's water come from?^[1]
• Are there any planets beyond Neptune? What is the explanation for the elongated orbits of a group of Kuiper belt objects?^[2]
• Rotation rate of Saturn: Why does the magnetosphere of Saturn exhibit a (slowly changing) periodicity close to that at which the planet's clouds rotate? What is the true rotation rate of Saturn's deep interior?^[3]

Stellar astronomy and astrophysics

• Solar cycle: 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?^[4] What caused the Maunder Minimum and other grand minima, and how does the solar cycle recover from a minimum state?
• Coronal heating problem: Why is the Sun's corona (atmosphere layer) so much hotter than the Sun's surface? Why is the magnetic reconnection effect many orders of magnitude faster than predicted by standard models?
• 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 – apparently regardless of the initial conditions?^[5]
• Supernovae: What is the exact mechanism by which an implosion of a dying star becomes an explosion?
• p-nuclei: What astrophysical process is responsible for the nucleogenesis of these rare isotopes?
• Fast radio bursts: Transient radio pulses lasting only a few milliseconds, from emission regions thought to be no larger than a few hundred kilometres, and estimated to occur several hundred times a day. While several theories have been proposed, there is no generally accepted explanation for them. The only known repeating FRB emanates from a galaxy roughly 3 billion light years from Earth.^[6][7]
• The Oh-My-God particle and other ultra-high-energy cosmic rays: What physical processes create cosmic rays whose energy exceeds the GZK cuttoff?^[8]
• Nature of KIC 8462852, commonly known as Tabby's Star: What is the origin of unusual luminosity changes of this star?

Cosmology

Estimated distribution of dark matter and dark energy in the universe

• Dark matter: What is the identity of dark matter?^[9] Is it a particle? Is it the lightest superpartner (LSP)? Do the phenomena attributed to dark matter point not to some form of matter but actually to an extension of gravity?
• Dark energy: What is the cause of the observed accelerated expansion (de Sitter phase) of the universe? 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? Is dark energy a pure cosmological constant or are models of quintessence such as phantom energy applicable?
• Baryon asymmetry: Why is there far more matter than antimatter in the observable universe?
• Cosmological constant problem: Why does the zero-point energy of the vacuum not cause a large cosmological constant? What cancels it out?^[10][11]
• Size and shape of the universe: The diameter of the observable universe is about 93 billion light-years, but what is the size of the whole universe? 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 inflaton 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?^[12]

Extraterrestrial life

• Is there other life in the Universe? Especially, is there other intelligent life? If so, what is the explanation for the Fermi paradox?^[13][14]
• Nature of Wow! signal: Was this singular event a real signal and, if so, what was its origin?^[15]

References

See also

• Lists of unsolved problems
• List of unsolved problems in physics