How Science Works - Facing The Challenges of New Information

Artistic rendering of the possible view from LHS 3154b towards its low mass host star. Given its large mass, LHS 3154b probably has a Neptune-like composition.

Credit: Penn State
Creative Commons

Discovery of planet too big for its sun throws off solar system formation models | Penn State University

Theists embrace certainty at the expense of truth because it feels safe in an uncertain world in which truth is subordinate to opinion. Scientists embrace doubt because it leads them closer to the truth in a world full of wonder and begging to be understood, where truth is supreme and leads opinion wherever it may go.

In other words, religion is unreasonable certainty; science is reasonable uncertainty.

So, if there is one thing theists hate, it's being challenged with new information. It's not just scientific information that shows how far removed from reality their founding myths, such as those in the Bible, are, but information that shows, for example, that several of the 'epistles' attributed to Paul were forgeries trading on his name to give them added authority, or that the description of his 'conversion' on the road to Damascus was actually a description of a temporal lobe epileptic seizure and that his reason for being there didn't make any sense in the geo-political and legal context of the times.

So heavily invested are most devout theists that inconvenient facts like these are not considered reasons to change their minds; they are considered reasons to look for excuses to deny and dismiss the evidence. There is a vast and very lucrative industry, especially in the USA, which specialises in selling people 'reasons' to deny the science and believe the myths in the Bible, for instance. Remaining stoically ignorant of science is even considered virtuous in some parts of the world where religions still have a strangle hold.

Contrast that with science, where the most exciting news is that someone has discovered something that means we need to revise our thinking and change our minds. With science, when the facts change, the intellectually honest change their minds; with religion, when the facts change, they try to change the facts because the beliefs are sacred.

An example of how science welcomes and embraces new evidence is the news that cosmologists have probably discovered a massive planet orbiting a sun that is too small for such a large planet orbiting so close to it. This might not sound dramatic but if confirmed, it will demand a reassessment of how we think planetary systems form.

What are the current theories of planetary formation? As of my last knowledge update in January 2022, several theories of planetary formation were widely accepted in the scientific community. However, keep in mind that the field of planetary science is dynamic, and new research may have emerged since then. As of my last update, here are the main theories:

1. Nebular Hypothesis:
   • This is the most widely accepted theory for the formation of the solar system. It suggests that planets form from the gravitational collapse of a rotating disk of gas and dust around a young star, known as a protostellar nebula.
2. Core Accretion Model:
   • Within the framework of the nebular hypothesis, the core accretion model explains how gas giants like Jupiter and Saturn form. It involves the gradual accumulation of solid particles to form a solid core, followed by the rapid accretion of gas onto this core.
3. Disk Instability Model:
   • This alternative model proposes that clumps of gas in the protostellar disk can become gravitationally unstable and collapse directly to form gas giants without the need for a solid core. While less favored than the core accretion model for explaining the formation of gas giants, it is still considered a viable possibility.
4. Pebble Accretion:
   • This theory suggests that small, centimeter-sized pebbles play a crucial role in the formation of planets. These pebbles can efficiently come together to form planetesimals, which then go on to form planets. Pebble accretion is often considered within the context of the core accretion model.
5. Grand Tack Hypothesis:
   • This hypothesis proposes that Jupiter and Saturn migrated inward toward the sun early in the solar system's history and then moved back outward. This migration may have influenced the formation and distribution of other planets, particularly the inner rocky planets.
6. Late Heavy Bombardment:
   • While not directly related to planetary formation, the Late Heavy Bombardment theory suggests that the solar system experienced a period of intense asteroid and comet impacts around 4 billion years ago. The precise causes and timing of this event are still debated.

These theories are not mutually exclusive, and aspects of each may contribute to our understanding of planetary formation. Ongoing observations, simulations, and laboratory experiments continue to refine and extend these models. It's recommended to check the latest scientific literature for the most up-to-date information on planetary formation theories.

Are the disk instability model and the gravitational instability models the same thing?

While the terms "disk instability model" and "gravitational instability model" might be used interchangeably in some contexts, they can refer to slightly different aspects of planetary or star formation. Let's break down the distinctions:

1. Disk Instability Model:
   • The disk instability model generally refers to a mechanism where instabilities within the protostellar disk lead to the formation of massive clumps directly. These clumps can then go on to become giant planets or even binary star systems. The disk instability model does not necessarily require the presence of a solid core, and the collapse occurs directly within the gas-rich disk.
2. Gravitational Instability Model:
   • Gravitational instability, in a broader sense, refers to any situation where a system becomes unstable due to its own gravity. In the context of planetary formation, this can include both the disk instability model and other scenarios where gravitational forces play a crucial role. For example, the term might be used to describe situations where spiral arms or density waves in the protostellar disk lead to the formation of protoplanetary clumps.

In summary, the disk instability model is a specific subset of the gravitational instability concept, focusing on the idea that the protostellar disk itself becomes gravitationally unstable and forms clumps. However, depending on the context, the terms might be used more broadly or interchangeably. It's always helpful to consider the specific details of the model being discussed to understand whether it involves disk instability, gravitational instability, or a combination of both.

The planet in question is thirteen times the mass of Earth, in other words, about the mass of Jupiter, orbiting a star that is only one ninth the mass of the sun. This means the mass ratio of the planet to its star is 100 times the mass ratio of Earth to the sun. The star, LHS 3154, is a cool, white dwarf star.

The problem for the accretion disk theory is that to form a planet this massive would require a dust-mass and dust-to-gas ratio of the accretion disk surrounding stars like LHS 3154 — when they were young and newly formed — 10 times higher than what was observed, based on measurements of the planet's core density.

The planet-forming disk around the low-mass star LHS 3154 is not expected to have enough solid mass to make this planet, but it’s out there, so now we need to reexamine our understanding of how planets and stars form.

What we have discovered provides an extreme test case for all existing planet formation theories. This is exactly what we built HPF to do, to discover how the most common stars in our galaxy form planets — and to find those planets.

Professor Suvrath Mahadevan, co-author
Verne M. Willaman Professor of Astronomy and Astrophysics
Penn State, Pennsylvania, PA, USA

The team found the planet/star system when looking for exoplanets with liquid water ( believe to be a basic requirement for life to evolve) using an instrument called the Habitable Zone Planet Finder (HPF) which was designed at Penn State.

Making the discovery with HPF was extra special, as it is a new instrument that we designed, developed and built from the ground-up for the purpose of looking at the uncharted planet population around the lowest mass stars. Now we are reaping the rewards, learning new and unexpected aspects of this exciting population of planets orbiting some of the most nearby stars.

Guðmundur Stefánsson, lead author
NASA Sagan Fellow in Astrophysics
Princeton University.

[Guðmundur Stefánsson] helped develop HPF and worked on the study as a graduate student at Penn State.

Based on current survey work with the HPF and other instruments, an object like the one we discovered is likely extremely rare, so detecting it has been really exciting. Our current theories of planet formation have trouble accounting for what we’re seeing.

Megan Delamer, co-author
Astronomy graduate student
Penn State

In the case of the massive planet discovered orbiting the star LHS 3154, the heavy planetary core inferred by the team’s measurements would require a larger amount of solid material in the planet-forming disk than current models would predict, Delamer explained. The finding also raises questions about prior understandings of the formation of stars, as the dust-mass and dust-to-gas ratio of the disk surrounding stars like LHS 3154 — when they were young and newly formed — would need to be 10 times higher than what was observed in order to form a planet as massive as the one the team discovered.

An artistic rendering of the mass comparison of LHS 3154 system and our own Earth and Sun.

Credit: Penn State Creative Commons

Sadly, the team's paper in Science is behind a paywall, but the abstract is available:

Abstract

Theories of planet formation predict that low-mass stars should rarely host exoplanets with masses exceeding that of Neptune. We used radial velocity observations to detect a Neptune-mass exoplanet orbiting LHS 3154, a star that is nine times less massive than the Sun. The exoplanet’s orbital period is 3.7 days, and its minimum mass is 13.2 Earth masses. We used simulations to show that the high planet-to-star mass ratio (>3.5 × 10−4) is not an expected outcome of either the core accretion or gravitational instability theories of planet formation. In the core-accretion simulations, we show that close-in Neptune-mass planets are only formed if the dust mass of the protoplanetary disk is an order of magnitude greater than typically observed around very low-mass stars.

Guðmundur Stefánsson et al.
A Neptune-mass exoplanet in close orbit around a very low-mass star challenges formation models.
Science 382, 1031-1035 (2023). DOI:10.1126/science.abo0233

© 2023 The Authors. Published by the American Association for the Advancement of Science.
Reprinted with kind permission under license #5680460158273.

Of course, there are other possible explanations, such as a captured 'wandering' planet which didn't form in an accretion disk around this star. It is also possible that the calculation of the mass of the planet, the sun or both is wrong, but that seems unlikely.

So what cosmologists are faced with is evidence inconsistent with current theories of planetary formation and the scientists who discovered it can scarcely conceal their excitement.

One thing creationists who cling to certainty at the expense of truth frequently feign bemusement over, is the fact that science books go out of date and need to be 'rewritten' (more correctly simply revised) in the next edition. This is because when the facts change, scientists change their minds, so the latest editions of the books are closer to the current consensus and closer to the truth than earlier editions.

By contrast, religious fundamentalists can't change their minds and so are stuck with the same 'science' that inspired the Bronze Age authors or their sacred texts and collected origin myths, which evidence-based science falsified long ago.

The ability to reassess the facts and incorporate new information is science's great strength; religion's inability to revise its thinking is its great weakness.

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