Could Modified Gravity Kill Planet Nine?

Original Article ↗ Hacker News Discussion ↗

Astrophysics enthusiasts weigh whether anomalies in the outer solar system are better explained by a hypothetical “Planet Nine” or by Modified Newtonian Dynamics (MOND), a proposal that tweaks gravity at very low accelerations. Many argue dark matter remains the stronger overall framework because it fits cosmological data like the cosmic microwave background and large-scale structure, while others counter that MOND elegantly accounts for galaxy rotation curves and certain empirical relationships with far fewer free parameters. Along the way, commenters probe what makes a theory scientific and falsifiable, how much “tuning” dark matter models require, and why detecting a distant, faint planet (or ruling it out) is so technically difficult.

MOND vs. Dark Matter

  • Several comments contrast MOND (a modified-gravity framework) with dark matter (DM).
  • Pro-DM arguments:
    • DM fits more data overall, especially baryon acoustic oscillations in the CMB and systems like the Bullet Cluster.
    • ΛCDM uses a small number of global parameters; galaxy-to-galaxy variation is treated as outcomes of structure formation, not free knobs.
  • Pro-MOND arguments:
    • MOND explains galaxy rotation regularities (e.g., Tully–Fisher, “Renzo’s rule”), external field effects, early galaxies, and some galaxies with little or no apparent dark matter.
    • MOND uses one main acceleration-scale parameter; critics say DM effectively uses many free parameters via flexible halo profiles.

Falsifiability and Theory Quality

  • Some argue MOND is not a single theory but a family; when one variant fails, another is proposed, which they see as “unfalsifiable,” akin to supersymmetry or string theory.
  • Others respond that specific MOND formulations are falsifiable; by contrast, unconstrained dark matter distributions can always be “painted on” to match dynamics unless DM is detected independently.
  • A recurring theme: a “good” theory should predict new phenomena, not just retrofit existing anomalies.

Planet Nine and Modified Gravity

  • The paper under discussion claims MOND effects from the galactic center could mimic the orbital clustering that motivated Planet Nine, potentially removing the need for such a body.
  • Some ask whether DM could produce similar inner–solar-system anomalies; quoted text from the paper implies “no,” but detailed reasoning is not fully unpacked in the thread.
  • Others question whether MOND should even be relevant so close to the Sun, given its usual “low-acceleration” framing; a reply notes the key acceleration is relative to the galactic center, not the Sun.

Detectability of Planet Nine

  • Several comments stress how hard it is to see a distant, cold planet: reflected sunlight scales roughly as distance⁻⁴.
  • Radar or active searches would face the same steep distance penalty and enormous power requirements.
  • Some note exoplanets are often easier to detect because they modulate already-bright stars (transits, Doppler shifts, microlensing), whereas Planet Nine is intrinsically faint against a crowded stellar background.

Gravity, Mercury, and Historical Analogies

  • A subthread compares the Planet Nine vs. MOND debate to the historical search for “Vulcan” vs. Einstein’s GR resolving Mercury’s perihelion precession.
  • One commenter offers a “relativistic mass” explanation for Mercury’s precession; multiple replies argue this is incorrect and that GR attributes the effect to spacetime curvature (an effective extra potential term), independent of Mercury’s own properties.
  • There is disagreement over whether certain intuitive explanations are merely alternative descriptions or actually wrong.

Dark Matter Skepticism

  • A minority equate DM to a modern “ether” with “zero evidence,” claiming it is mainly a name for mismatches between model and observation.
  • Others counter that there are “mountains of evidence” from many independent probes; what’s missing is identification of the particle or object type, not gravitational effects themselves.

Interstellar Objects and “Empty” Space

  • One line of discussion considers whether interstellar space might be filled with undetected planet-sized or smaller bodies.
  • Replies point to gravitational microlensing surveys and constraints on compact objects (“MACHOs”), suggesting upper limits on how much such material can contribute to DM, though small, very dark bodies could still be numerous without changing the overall “emptiness” of space.
  • Rogue planets and a very extended Oort cloud are mentioned as examples of known or plausible non-luminous mass that is still gravitationally minor on galactic scales.

Science Communication and YouTube

  • A linked video critique of MOND and of the specific paper is discussed.
  • Several commenters emphasize checking cited papers directly and treating YouTube explanations as entry points rather than final authority.
  • There is some concern about “commentator sports” dynamics in physics outreach, but also appreciation for communicators who stay tied to the peer-reviewed literature.

Planet Definitions and Naming

  • Side discussion on Pluto’s demotion and Planet Nine vs. “Planet X” terminology.
  • Some argue common language need not follow IAU definitions and prefer the older, more evocative names; others clarify that “Planet Nine” and “Planet X” have specific historical and technical meanings.