Published 09/2019- updated 09/26
Comets present an interesting problem for an ancient solar system. Each time an active comet passes close to the Sun, solar heating turns frozen material in its nucleus into gas, carrying dust and other material into space. The spectacular coma and tail we observe are therefore visible evidence that the comet is losing material.
NASA acknowledges that this process limits the lifetime of active comets. In discussing short-period comets, NASA states that they “evaporate so quickly compared to the age of the solar system that we shouldn’t see any.”¹ Yet short-period comets are still present throughout our solar system.
Perhaps the most famous example is Halley’s Comet. It was last visible from Earth in 1986 and returns approximately every 76 years. NASA estimates its nucleus to be approximately 9.3 by 5 miles and reports that a comet the size of Halley loses approximately 3 to 10 feet (1 to 3 meters) of material from its surface during each orbit around the Sun.²
A scientific analysis published in Monthly Notices of the Royal Astronomical Society estimated that Halley lost approximately 2.8 × 10¹⁴ grams of material during its 1910 appearance. The author estimated that Halley could disappear after roughly another 2,300 close passages of the Sun.³
At an average orbital period of approximately 76 years, 2,300 additional passages correspond to only about 175,000 years.
Whatever an individual comet’s exact lifetime, the important point is the enormous difference in scale. Active short-period comets cannot simply continue making close approaches to the Sun for 4.6 billion years. Their active lifetimes are vastly shorter than the conventional age assigned to the solar system.
NASA agrees with this basic observation. It states that an average periodic comet may survive approximately 1,000 trips around the Sun before eventually exhausting its volatile material, becoming dormant, breaking apart, or otherwise ceasing to behave as an active comet.²
The conventional explanation is not that individual active comets have survived billions of years of repeated passages near the Sun. Instead, astronomers propose reservoirs of icy bodies far from the Sun that continually supply replacement comets to the inner solar system.
For short-period comets, the principal source is thought to be the Kuiper Belt and related trans-Neptunian populations. Unlike the Oort Cloud, astronomers can directly observe the Kuiper Belt. Thousands of trans-Neptunian objects have been discovered, providing observational support for a large population of icy bodies beyond Neptune.⁴ Gravitational interactions, particularly involving Neptune and the other giant planets, can alter some of these objects’ orbits and eventually send them toward the inner solar system.
Long-period comets present a different situation.
To explain the apparent source of long-period comets arriving from many different directions, astronomers propose an enormous spherical reservoir of icy objects surrounding the solar system called the Oort Cloud.
NASA describes the Oort Cloud as a “hypothesized spherical reservoir of comet nuclei” and acknowledges:
“No confirmed direct observations of the Oort cloud have been made yet.”⁵
The proposed cloud would exist extraordinarily far from the Sun, with its distant portions extending tens of thousands of astronomical units away. Its existence is inferred largely from the orbital distribution of long-period comets rather than from direct observation of the proposed population itself.
NASA similarly explains that the Oort Cloud “still remains a theory because the innumerable comets that make it up are too faint and distant to be directly observed.”⁶
This distinction is important. Astronomers are not claiming that today’s active comets have repeatedly passed near the Sun for 4.6 billion years. Their short lifetimes make that impossible. Instead, the ancient-solar-system model requires large reservoirs of cometary bodies capable of supplying new comets over billions of years.
The short lifetime of an individual comet does not, by itself, establish the age of the entire solar system. An ancient solar system can contain young active comets if older reservoirs continually inject previously inactive icy bodies into cometary orbits.
But that means the existence of comets creates a genuine replenishment requirement for any billions-of-years model.
For short-period comets, astronomers point primarily to the observed Kuiper Belt and related populations. For long-period comets, they invoke the much more distant Oort Cloud, whose population has not been directly observed.
The observational fact remains remarkably simple: active comets are temporary objects. They lose material every time they approach the Sun, and many survive only a tiny fraction of the conventional 4.6-billion-year age of the solar system.
Therefore, the existence of active comets today requires an explanation. The conventional model supplies that explanation through continuing delivery from distant reservoirs. The Kuiper Belt is observed; the enormous Oort Cloud population proposed to supply long-period comets remains an inference from cometary orbits.
Comets themselves therefore do not function as billion-year clocks. What we actually observe are comparatively short-lived objects whose continued presence in an ancient solar system requires continual replenishment.
SOURCES
NASA Science, “Hubble Confirms Largest Comet Nucleus Ever Seen.” NASA describes the Oort Cloud as remaining theoretical because its proposed constituent bodies are too faint and distant to observe directly.
NASA, “Kuiper Belt: In Depth.” NASA notes the rapid evaporation of short-period comets relative to the conventional age of the solar system.
NASA Science, “1P/Halley.” NASA gives Halley’s dimensions, approximately 76-year orbital period, estimated surface loss of 1–3 meters per orbit, and discussion of periodic-comet lifetimes.
David W. Hughes, “The Size, Mass, Mass Loss and Age of Halley’s Comet,” Monthly Notices of the Royal Astronomical Society 213 (1985): 103–109. The paper estimated Halley’s mass loss and approximately another 2,300 close perihelion passages before disappearance.
NASA Science, “Kuiper Belt: Facts.” NASA describes the observed population of icy bodies beyond Neptune and the Kuiper Belt’s proposed relationship to short-period comets.
NASA Science, “The Solar System,” Basics of Space Flight. NASA describes the Oort Cloud as a hypothesized reservoir and states that no confirmed direct observations have yet been made.
