Dark matter is a confounding concept that teeters on the leading edges of cosmology and physics. We don’t know what it is or how exactly it fits into our understanding of the Universe. We only know that its unseen mass is a critical part of the cosmos.
Astronomers know dark matter exists. They can tell by the way galaxies rotate, by exploiting gravitational lensing, and by analyzing fluctuations in the Cosmic Microwave Background. But new research suggests that there might be another way to detect its presence.
The research is “Dark Matter (S)pins the Planet,” and it’s available at arxiv.org. Haihao Shi, from the Xinjiang Astronomical Observatory at the Chinese Academy of Sciences, is the lead author. The co-authors are all from Chinese research institutions.
“Dark matter(DM) constitutes approximately 85% of the Universe’s matter content, as evidenced by a multitude of astrophysical and cosmological observations. Despite its pervasive presence, the fundamental nature and composition of dark matter particles remain elusive, pointing to new physics beyond the Standard Model and general relativity,” the authors write, capturing the issue neatly.
The research rests on previous research suggesting that DM can find its way into the interior of planets, a phenomenon called dark matter planetary capture. The idea is that the gravitational pull of planets can attract dark matter particles. The physics behind it are complex, but scientists are working on it, including trying to estimate its density inside planets. Scientists expect the density to be very low, making it difficult to detect.
There are a variety of explanations for what DM could be. It could be primordial black holes, it could be axions, or it could be Weakly Interacting Massive Particles (WIMPs). There are other candidates as well. But this research tries to constrain DM’s properties on a planetary scale rather than on a microscopic or cosmic scale.
“Planets serve as long-term probes of dark matter effects, having interacted with the surrounding dark matter halo for billions of years,” the authors write. “These interactions may lead to cumulative, observable consequences, such as changes in planetary temperature, rotational dynamics, and atmospheric properties.”
The basic idea behind dark matter capture is that DM interacts with planetary matter and deposits energy into the planet. While dark matter and baryonic matter don’t interact or collide in the normal sense, they can interact due to factors like quantum tunnelling. These interactions lead to changes in a planet’s rotation and temperature, accelerating one and raising the other. Scientists can observe these changes, and the authors have developed a new approach to detecting them.
This illustration shows WIMPs annihilating one another and releasing gamma rays. When these theoretical particles annihilate each other in a planet’s interior, they heat the planet. Image Credit: Greg Stewart/SLAC National Accelerator Laboratory
Though the physics themselves are fascinating on their own, planetary dark matter could affect habitability. “Furthermore, dark matter effects at planetary scales could influence planetary habitability by altering thermal conditions, potentially affecting the stability of liquid water and atmospheric evolution,” the paper states.
When dark matter particles enter planets, they are subject to scattering, capture, and annihilation. When they scatter—or collide—they impart kinetic energy into the planetary particles, which manifests as heat. The same happens when they’re annihilated. The temperature increase is due to how much dark matter enters the planet, and the energy that’s input can also accelerate the planet’s rotation period.
WIMPs are a strong candidate for explaining dark matter. Inside planets, WIMPs can theoretically annihilate one another, releasing energy that heats the planet. Credit: NASA Scientific Visualization Studio.
In their work, the researchers simulated dark matter planetary capture in 15 confirmed exoplanets, including ones like 55 Cancri d (Lipperhey) and Epsilon Eridani b, both of particular interest to scientists. They also applied their model to Jupiter and Earth.
“Our theory suggests that the energy provided by dark matter heating is not entirely converted into temperature but is distributed according to the planet’s intrinsic properties, such as mass and radius, as well as its current state, including temperature and angular velocity,” the authors write.
Earth is not immune to dark matter capture, according to the research.
There could be dark matter captured in Earth’s interior, where, due to quantum effects, it interacts with baryonic matter. Image Credit: ESA
The authors write that “… our predictions suggest that the combined energy input from dark matter and the Sun will lead to a surface atmospheric temperature increase of approximately 0.015 K over 100 years and 0.15 K over 1000 years.”
Dark matter heating can also increase a planet’s rotational velocity, though that’s more difficult to differentiate from other influences. Things like tidal effects and Earthquakes can also affect rotational velocity. “For Earth, we predict that the heating of dark matter will accelerate its rotation period on the order of seconds per hundred years,” they write. Earth’s rotation period will decrease by about 12 seconds in 100 years. In 1,000 years, that’s 120 seconds. These are large numbers, and the authors say we should be able to detect these effects with ground-based measurement methods.
The authors claim that as we understand these effects more clearly, they could help us understand exoplanet habitability.
“In the future, as humanity searches for a second home in the universe, the impact of dark matter on planetary rotation proposed in this work may serve as a reference for assessing planetary habitability,” they conclude.