In the past few decades, the number of planets discovered beyond our solar system has grown to thousands. Currently 4,389 exoplanets have been confirmed in 3,260 systems, and a further 5,941 candidates are awaiting confirmation. Thanks to numerous observations and studies, scientists have learned a lot about the types of planets that exist in our universe, how planets form, and how they evolve.
An important consideration in this is how planets become (and remain) habitable over time. In general, astrobiologists have worked on the assumption that habitability is due to where a planet orbits within a system – within the habitable zone (HZ) of its parent star. However, new research by a team at Rice University shows that where a planet forms in its respective star system could be just as important.
The study, recently published in Nature Geoscience, was led by Rice graduate student Damanveer Grewal, in which several colleagues from the Institute of Earth, Environmental and Planetary Sciences at Rice University participated (including Rajdeep Dasgupta, the Maurice Ewing professor of Earth system science in rice). Together they looked beyond the goldilocks zone of the stars to see how factors involved in planet formation would ultimately affect habitability.
A study by scientists at Rice University shows that the formation of a planet in a star system plays a crucial role in its habitability. Photo credit: Rice University / Amrita P. Vyas
Basically, a star’s HZ (or Goldilocks Zone) refers to the region where an orbiting planet experiences conditions warm enough to support liquid water on its surface and a rich atmosphere – the main ingredients for life. After considering the elements that go into the planetary formation, Grewal and his colleagues concluded that the amount of volatile elements a planet captures and retains during formation also determines whether it becomes habitable.
The focus is on the time it takes for the material to evolve from a circumsolar disk to a protoplanet, and the time it takes for the protoplanet to move into its various layers (a metal core, a silicate mantle, and a crust, as well as an atmospheric one Envelope) to differentiate. The balance between these two processes is critical in determining which volatile elements a rocky planet will hold back, especially nitrogen, carbon, and water, which give rise to life.
Using Dasgupta’s high pressure laboratory in Rice, the research team used nitrogen as a proxy for volatiles and simulated how protoplanets are differentiated. They found that during this process, most of a protoplanet’s nitrogen is lost from the mantle and escaped into the atmosphere. From there, the nitrogen is lost to space when the protoplanet either cools down or collides with other celestial objects in the next phase of its growth.
However, if the metallic core retains enough nitrogen, it could still be so significant that over time it will help to later form an “Earth-like” atmosphere (where it plays an important role as a buffer gas). From this, the researchers were able to model the thermodynamics and its effects on the nitrogen distribution between the atmosphere of a protoplanet, the molten silicon dioxide layers and the core.
Artist’s impression of the diversity of habitable zones for different types of stars. Photo credit: NASA / Kepler Mission / Dana Berry
As Grewal stated in a Rice University press release:
“We simulated high-pressure temperature conditions by exposing a mixture of nitrogen-containing metal and silicate powders to almost 30,000 times atmospheric pressure and heating them above their melting points. Small lumps of metal embedded in the silicate glasses of the obtained samples were the respective analogues of protoplanetary cores and mantles. “
“We found that the fractionation of nitrogen between all these reservoirs is very sensitive to the size of the body. With this idea we could calculate how nitrogen would have separated over time between different reservoirs of protoplanetary bodies to eventually build a habitable planet like Earth. “
Of course, this research has implications for our understanding of how the earth was formed over 4.5 billion years ago. Their results indicate that material from the protoplanetary disk quickly accumulated and formed a planetary embryo the size of a Moon or Mars before it completed the differentiation process and assumed its current metallic core, silicate mantle / crust, and gaseous envelope arrangement.
For the entire solar system, they estimate that planetary embryos formed within 1-2 million years after the sun and that the remaining nebula formed into a disk surrounding them – much earlier than previously assumed. If the rate of differentiation were faster than the rate of accretion for these embryos, none of the rocky planets would have accreted enough volatile matter and the earth would not have developed the conditions necessary for life.
The artist’s concept of a collision between Proto-Earth and Theia is said to have occurred 4.5 billion years ago. Photo credit: NASA
Dasgupta is not only Professor of Earth System Science at Rice, but also the principal researcher of the CLEVER planets. This NASA-funded collaborative project examines how vital elements could have come together on rocky planets across the cosmos. As he summarized:
“Our calculations show that the formation of an earth-sized planet via planetary embryos, which grew extremely quickly before metal-silicate differentiation was performed, is a unique way of meeting the Earth’s nitrogen budget. This work shows that nitrogen has a much greater affinity for nucleating metallic liquids than previously assumed. “
This latest research builds on previous findings by Grewal and Dagusta (and colleagues), such as a 2019 study that showed how much of the earth’s volatile content may be due to the effects of the moon. This was followed by research published in 2021, which indicated that the Earth was extracting more nitrogen from local sources in the solar system than previously thought.
“We have shown that protoplanets growing in both inner and outer regions of the solar system accumulate nitrogen and the earth gets its nitrogen from these two regions by accumulating protoplanets,” said Grewal of this study, which was published on Jan. 21 Appeared in 2021. Edition of natural astronomy. “However, it was not known how the Earth’s nitrogen budget was set.”
These findings could have a significant impact on future research into how planetary systems form, evolve, and eventually evolve the ability to support life. In the years to come, robotic missions exploring the solar system’s oldest objects (near-Earth, main belt, and Trojan / Greek asteroids) could provide additional insight into its early history – a time when the seeds of life-giving elements were planted on Earth and other planets .
Further reading: Rice University, Nature
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