Neptune Is Rapidly Cooling And Scientists Have No Idea Why

MARCH 9, 2023

A research study from researchers at the University of Leicester has actually revealed that Neptune, the solar system’s most distant world, except for the dwarf planets like Pluto, is going through an unanticipated cooling even though the world has actually entered its summertime season. Neptune is 30 times farther away from the Sun than Earth, and likewise has an axial tilt that triggers it to experience seasons. One Neptune year is about 165 Earth years, and its seasons are each over 40 Earth years.

The research study, led by a global group of scientists consisting of researchers from Leicester and NASA’s Jet Propulsion Laboratory (JPL), evaluated all existing ground-based imaging of Neptune in the mid-infrared from several observatories from 2003-2020.

As Neptune moved into its southern summer season, the researchers observed its typical planetwide temperature levels fall by 14 degrees Fahrenheit (8 degrees Celsius) between 2003 and 2018.
“This change was unexpected,” said Dr. Michael Roman, a postdoctoral research associate at the University of Leicester and lead author on the new paper. “Since we have been observing Neptune during its early southern summer, we would expect temperatures to be slowly growing warmer, not colder.”
Observations of Neptune have actually just been a possibility for the past couple of years, so the information just shows part of a Neptune season.

Dr. Glenn Orton, the Senior Research Scientist at JPL and co-author of the research study, discussed, “Our data covers less than half of a Neptune season, so no one was expecting to see large and rapid changes.”

The reason for the unforeseen temperature level modifications is unidentified, however, the researchers believe the Sun’s 11-year solar cycle might contribute.
“The temperature variations may be related to seasonal changes in Neptune’s atmospheric chemistry, which can alter how effectively the atmosphere cools,” Roman said. “But random variability in weather patterns or even a response to the 11-year solar activity cycle may also have an effect.”
In the research study, the researchers keep in mind that Neptune’s temperature level approximately followed the pattern in solar activity determined utilizing the Lyman-alpha (Lyα)* line, a method of finding modifications in solar activity.

“A strong solar maximum occurred in ∼2001 (cycle 23), followed by a drop in the Lyα over the subsequent 10 yr,” the study says. “The deep minimum in 2009 (cycle 24) gave way to a weak solar maximum in ∼2014. Currently, solar cycle 25 is rebounding from another deep minimum in 2019. Plotted with the time series of Lyα, Neptune’s mid-IR radiances followed a roughly similar trend.”

Researchers anticipate that the James Webb Space Telescope, the most effective telescope ever launched into the area, will assist address these and other concerns about Neptune and its next-door neighbor, the ice giant Uranus.

“The exquisite sensitivity of [JWST’s] mid-infrared instrument, MIRI, will provide unprecedented new maps of the chemistry and temperatures in Neptune’s atmosphere, helping to better identify the nature of these recent changes,” stated Leigh Fletcher, Professor of Planetary Science at the University of Leicester and a co-author of the brand-new research study.


* Lyman α radiation is a kind of ultraviolet light coming from hydrogen. Lyman-alpha line, typically denoted by Ly-α, is a spectral line of hydrogen (or, more generally, of any one-electron atom) in the Lyman series. It is emitted when the atomic electron transitions from an n = 2 orbital to the ground state (n = 1), where n is the principal quantum number. In hydrogen, its wavelength of 1215.67 angstroms (121.567 nm or 1.21567×10−7 m), corresponding to a frequency of about 2.47×1015 Hz, places Lyman-alpha in the ultraviolet (UV) part of the electromagnetic spectrum.

Lyman alpha systems are becoming a very useful source of information in physical cosmology.

The Lyman series is the series of energies required to excite an electron in hydrogen from its lowest energy state to a higher energy state.
hydrogen levels
The case of particular interest for cosmology is where a a hydrogen atom with its electron in the lowest energy configuration gets hit by a photon (light wave) and is boosted to the next lowest energy level. The energy levels are given by En = -13.6 eV/n2 and the energy difference between the lowest (n=1) and second lowest(n=2) levels corresponds to a photon with wavelength 1216 angstroms. The reverse process can and does occur as well, where an electron goes from the higher n=2 energy state to the ground state, releasing a photon of the same energy.

The absorption or emission of photons with the correct wavelength can tell us something about the presence of hydrogen and free electrons in space. That is, if you shine a light with wavelength 1216 at a bunch of neutral hydrogen atoms in their ground state, the atoms will absorb the light, using it to boost the electron to a higher energy state. If there are a lot of neutral hydrogen atoms in their ground state, they will absorb more and more of the light. So if you look at the light you receive, intensity as a function of wavelength, you will see a dip in the intensity at 1216 angstroms, depending on the amount of neutral hydrogen present in its ground state. The amount of light absorbed ('optical depth') is proportional to the probability that the hydrogen will absorb the photon (cross section) times the number of hydrogen atoms along its path.

qso system
Because the universe has many high energy photons and hydrogen atoms, both the absorption and emission of photons occurs frequently. In Lyman alpha systems, the hydrogen is found in regions in space, and the source for the photons are quasars (also called qsos), very high energy light sources, shining at us from behind these regions.

Because the universe is expanding, one can learn more than just the number of neutral hydrogen atoms between us and the quasar. As these photons travel to us, the universe expands, stretching out all the light waves. This increases the wavelengths lambda and lowers the energies of the photons (`redshifting').

Neutral hydrogen atoms in their lowest state will interact with whatever light has been redshifted to a wavelength of 1216 angstroms when it reaches them. The rest of the light will keep travelling to us.

It is common to see a series of absorption lines, called the Lyman alpha forest. Systems which are slightly more dense, Lyman limit systems, are thick enough that radiation doesn't get into their interior. Inside these regions there is some neutral hydrogen remaining, screened by the outer region layers. If the regions are very thick, there is instead a wide trough in the absorption, and one has a damped Lyman alpha system. Absorption lines generally aren't just at one fixed wavelength, but over a range of wavelengths, with a width and intensity (line shape) determined in part by the lifetime of the excited n=2 hydrogen atom state. These damped Lyman alpha systems have enough absorption to show details of the line shape such as that determined by the lifetime of the excited state. These dense clumps are thought to have something to do with galaxies that are forming.



We learn several things from these systems, including the following:
  • Neutral hydrogen: because we see any light at all, we can limit how much neutral hydrogen is out there between us and the quasar and what its distribution is. It used to be thought that there was a smooth intergalactic medium (IGM) with regions embedded in it, and the smooth background would provide an absorption at all positions between us and the quasar (Gunn-Peterson effect). But observers only see evidence of lumpy regions. There isn't evidence for a spatially smooth component of neutral hydrogen between us and the quasar sources. It is a question of active research what is making the amount of neutral hydrogen so small (that is, what is ionizing the rest of the hydrogen).
  • ly alpha simulation
  • Structure formation: the regions in the Lyman alpha systems are not very massive compared to objects like galaxies. As a result, reliable computer simulations (numerical experiments) of their gravitational collapse (formation) from primoridal fluctuations are possible. Until the 90's, it was thought that gravity alone could not form all the structure, in particular the filaments, walls, and voids which we observe. However, the Lyman alpha simulations did produce this structure by starting with small fluctuations in matter density and then letting gravity and other known forces act. It was not necessary to add other mechanisms to get the observed structure in these systems. The detailed properties of the structure and distribution of matter are frontiers of current research. At right is a picture of the distribution of neutral hydrogen found in simulations.

  • Hot dark matter: The numerical simulations have also shown, via their successful reproduction of properties of the regions, that one cannot have too much hot dark matterif one wants to agree with observations. (Too much hot dark matter erases structure on small scales.)

  • Distribution of matter: The Lyman alpha regions are formed by gas falling into gravitational potential wells of all the matter, not just the luminous matter. So they provide another tracer of dark matter.

  • Nucleosynthesis: deuterium is produced in 'the first three minutes' in the early universe, and afterwards is believed to only be destroyed. The Lyman alpha systems have deuterium in them too, and as these systems also have low amounts of metals (heavier elements), one might hope that they are measuring unprocessed or primordial deuterium. The deuterium also absorbs light from quasars, and thus its abundance can be measured in a similar way. Searches in many of these systems have provided the current strongest constraint on the amount of primoridal deuterium and thus the baryon density in the universe. See this link or this posteron big bang nucleosynthesis.

  • Cosmological constant: the path back to a given redshift depends on how the universe has expanded since that time. The angular extent of an object at any redshift also depends on the expansion of the universe since the light was emitted, but in a different way. Thus one can compare angular and radial (more precisely redshift) lengths for objects. If one knows the expected ratio of these lengths for an object for other reasons, one can constrain the expansion history of the universe, in particular the cosmological constant. A variant of this Alcock Pacynzski test involves tracing lines of sight through the Lyman alpha regions for neighboring quasars.

We have so much more to learn about our own solar system and its planets. New research led by space scientists at the University of Leicester has revealed how temperatures in Neptune's atmosphere have unexpectedly fluctuated over the past two decades. Scientists from Leicester and NASA's Jet Propulsion Laboratory (JPL), combined all existing thermal infrared images of Neptune gathered from multiple observatories over almost two decades. These include the European Southern Observatory's Very Large Telescope and Gemini South telescope in Chile, together with the Subaru Telescope, Keck Telescope, and the Gemini North telescope, all in Hawai'i, and spectra from NASA's Spitzer Space Telescope. By analysing the data, the researchers were able to reveal a more complete picture of trends in Neptune's temperatures than ever before.


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