August 5, 2020 | David F. Coppedge

Star Warts: Planets Rebel Against Theory

Things in space don’t always cooperate with theoretical expectations.

Phosphorus-rich stars with unusual abundances are challenging theoretical predictions (Nature Communications). Phosphorus is essential for life (especially in DNA), but Earth has too much phosphorus for theory. Seven astronomers in Spain describe the conundrum, beginning with a restatement of the theory that the consensus believes must be true:

Our DNA and proteins depend on phosphorus compounds.

Almost all chemical elements have been made by nucleosynthetic reactions in various kind of stars and have been accumulated along our cosmic history. Among those elements, the origin of phosphorus is of extreme interest because it is known to be essential for life such as we know on Earth. However, current models of (Galactic) chemical evolution under-predict the phosphorus we observe in our Solar System. Here we report the discovery of 15 phosphorus-rich stars with unusual overabundances of O, Mg, Si, Al, and Ce.

In order to save their secular theory, they have to steal some extra phosphorus from elsewhere:

Phosphorus-rich stars likely inherit their peculiar chemistry from another nearby stellar source but their intriguing chemical abundance pattern challenge the present stellar nucleosynthesis theoretical predictions. Specific effects such as rotation or advanced nucleosynthesis in convective-reactive regions in massive stars represent the most promising alternatives to explain the existence of phosphorus-rich stars. The phosphorus-rich stars progenitors may significantly contribute to the phosphorus present on Earth today.

It “may” – or it may not. That’s just a story to rescue a theory that is out of sync with observations. The take-home message is: Earth is an anomaly according to secular theory, thank God.

Most exoplanets are nothing like Earth. Credit: University of Warwick/Mark Garlick

Surprisingly Dense Exoplanet Challenges Planet Formation Theories (NSF NOIRLab). Another theory out of sync with observations concerns how planets form. K2-25B is a planet slightly smaller than Neptune, and it orbits an M-dwarf star. This planet is “unusual” say these astronomers at the National Science Foundation’s National Optical-Infrared Astronomy Research Laboratory (NOIRLab).

Astronomers predict that giant planets form by first assembling a modest rock-ice core of 5–10 times the mass of Earth and then enrobing themselves in a massive gaseous envelope hundreds of times the mass of Earth. The result is a gas giant like Jupiter. K2-25b breaks all the rules of this conventional picture: with a mass 25 times that of Earth and modest in size, K2-25b is nearly all core and very little gaseous envelope. These strange properties pose two puzzles for astronomers. First, how did K2-25b assemble such a large core, many times the 5–10 Earth-mass limit predicted by theory? [2] And second, with its high core mass — and consequent strong gravitational pull — how did it avoid accumulating a significant gaseous envelope?

Of K2-25B, they say, “this exoplanet’s existence is at odds with the predictions of leading planet formation theories.” And yet it does exist.

VLBA Finds Planet Orbiting Small, Cool Star (National Radio Astronomy Observatory). The NRAO found a Saturn-sized planet orbiting a small, cool star. This was only the second planet detected using a technique with radio telescopes that requires ultra-precise measurements of the “wobble” a planet makes on its parent star. They didn’t expect what they found:

“Giant planets, like Jupiter and Saturn, are expected to be rare around small stars like this one, and the astrometric technique is best at finding Jupiter-like planets in wide orbits, so we were surprised to find a lower mass, Saturn-like planet in a relatively compact orbit. We expected to find a more massive planet, similar to Jupiter, in a wider orbit,” said Salvador Curiel, of the National Autonomous University of Mexico. “Detecting the orbital motions of this sub-Jupiter mass planetary companion in such a compact orbit was a great challenge,” he added.

Ultima Thule, later renamed Arrokoth, was surprisingly smooth and looked like a snowman.

The sublimative evolution of (486958) Arrokoth (Icarus). This body in the Kuiper Belt of our solar system, photographed by the New Horizons spacecraft in 2019 flyby, surprised astronomers with its dual lobes and smooth surface. Six astronomers from six institutions are trying to put this humpty-dumpty asteroid together with theory. They propose two eons: one in which it was outgassing volatiles that blanketed its surface, and a second “quiescent” period.

These results suggest that the observed surface of Arrokoth is not primordial, but rather dates from the Quiescent Period. By contrast, the inability of sublimative torques to meaningfully alter Arrokoth’s rotation state suggests that its shape is indeed primordial, and its observed rotation is representative of its spin state after formation.

Surprisingly, the “vigorous” outgassing didn’t change Arrokoth’s orbit or spin. It did, however, change the spin of the astronomers’ report. They needed to spin a story in order to explain another unusual body.

We think laypeople should know what theoreticians do behind the scenes. Theories are not as neat as textbooks and magazines make them out to be.


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