A New Twist On Planet Formation

This image of the nearby young star TW Hydrae reveals the classic rings and gaps that signify planets are being formed in this protoplanetary disk. {ESO, Atacama Large Millimeter/submillimeter Array (ALMA)}

Before the first exoplanets were discovered in the 1990s,  our own solar system served as the model for what solar systems looked like.  The physical and chemical dynamics that formed our system were also seen as the default model for what might have occurred in solar systems yet to be found.

As the number of exoplanets identified ballooned via the Kepler Space Telescope and others, and  it became clear that exoplanets were everywhere and orbiting most every star, the model of our own solar system became obviously flawed.  The first exoplanet identified, after all, was a “hot Jupiter” orbiting very close to its star — a planetary placement previously thought to be impossible.

With the growing number of known exoplanets and their most unusual placements, the field of planet formation — focused earlier on understanding on how the planets of our system came into being and what they were made of — expanded to take in the completely re-arranged planetary and solar system menagerie being found.

This was basic science seeking to understand these newfound worlds, but it also became part of the fast-growing field of astrobiology, the search for planets that might be habitable like our own.

In this context, planet formation became associated with the effort to learn more about the dynamics that actually make a planet habitable — the needed composition of a planet, the nature of its Sun, its placement in a solar system and how exactly it was formed.

So the logic of planet formation became the subject of myriad efforts to understand what might happen when a star is born, surrounded by a ring of gas and dust that will in time include larger and larger collections of solids that can evolve into meteors, planetesimals and if all goes a particular way, into planets.

A thin section of primitive meteorite under a microscope. The various colors suggest different minerals that comprise meteorites. The round-shaped mineral aggregates are called chondrules, which are among the oldest known materials in our solar system. (Science)

As part of this very broad effort to understand better how planets form, meteorites have been widely used to learn about what the early solar system was like. Meteorites are from asteroids that formed within the first several million years of planetary accretion. If found quickly after falling to Earth,  meteorites have not been transformed by the ever-present geological and biological forces at work on here.

A 2017 paper on the isotopic composition of meteorites found, for instance, that roughly 90 percent of the matter that makes up the Earth came from the inner solar system while about 10 percent came from beyond Jupiter.

That study, however, looked only at isotopes of elements in the meteorites that withstood very high temperatures before turning into vapor — the “refractory” elements. Since the solar nebula from which our Sun and solar system originated was thought to be very hot, little attention has been paid to the elements that turn into vapor at lower temperatures because all dust carriers were assumed to be vaporized and homogenized.

These “volatiles” include hydrogen, carbon, nitrogen and more and some elements such as potassium and zinc that are considered “moderate volatiles” because of the not-too-high and not-too-low temperatures at which they vaporize.

But now two teams have pioneered the study of those volatile elements in meteorites and by extension in the early solar system. Volatiles, and moderate volatiles, are especially important in astrobiology because they are chemicals needed for the origin of life.

Nicole Nie and a team at the Carnegie Institution of Science just published a paper in the journal Science that describes how they identified the moderate volatile potassium in 32 meteorites.  Another team at Imperial College, London had a companion paper about detecting the moderate volatile zinc in meteorites.

“It is very exciting that we were able to find these isotopic anomalies in these volatiles because they were not previously expected,” said Nie, a postdoctoral researcher who will soon become an assistant professor at MIT.  “And they have already shown us important new understandings of our solar nebula.”

The inner solar system extends to past Mars and the outer solar system is beyond. Much of the material that formed Earth came from the inner solar system. (spaceplace.nasa.gov)

The process by which the two teams identified the previously undetected isotopic anomalies in volatiles in meteorites involves an elegant (if also complex) use of basic cosmic forces at play since the early times of our universe.

As Nie explained, it is well known now that at the time of the Big Bang the elements present in the universe were hydrogen, then helium and lithium.  All the other “heavy” elements and their isotopes were formed in massive stars, among which many went supernovae at the ends of their lives, when they ejected materials into interstellar space.

Nie is an isotope geo/cosmochemist, using the chemical and isotopic compositions of extraterrestrial materials to understand the history and diversity of the formation of our solar system. Following postdoctoral work at the Carnegie Institution of Science and Caltech, she will join MIT as an assistant professor this summer.

During those enormously explosive events, the heavier elements came into being via nuclear fission and fusion reactions.  And when they did, the elements were formed in different isotopic forms — i.e., with the same number of protons in their nuclei but different numbers of neutrons.  Potassium, for instance, has more than 20 isotopes, though only 3 are stable or long-lived.

The small variations in isotope abundances created by star explosions are called “nucleosynthetic anomalies” and were carried by interstellar dusts from many, many light-years away to our solar nebula and then solar system.

During the formation of the solar system, Nie said, interstellar dusts carrying these nucleosynthetic anomalies were incorporated into solids that condensed from the gas phase of the solar nebula, then into meteorites and the terrestrial planets, including Earth.

Different nucleosynthetic anomalies were present in meteorites formed in different parts of the early solar system. The origin of the material that formed Earth can then be determined by measuring the nucleosynthetic anomalies of meteorites.

This was done for non-volatile elements in 2017, with that estimate of 90 percent of the Earth’s mass from the inner solar system and 10 percent from the outer.  Now we have data for the potassium and zinc volatiles that make up Earth, and the percentage coming from the inner and outer solar systems is about the same.

Not only was the Earth’s volatile element budget a significant step forward, but Nie says equally important is that the results of potassium and zinc  studies show that the pre-solar dust carriers of volatile elements survived the hot solar nebula.

This suggests that the solar nebula was not a homogeneous collection of gas and dust but rather was quite heterogeneous. There were some areas, in particular in the outer solar system, that had temperatures low enough to preserve the carriers of volatile elements.

These findings overturn a long-held idea that the heat of the nebula destroyed any nucleosynthetic anomalies of volatile elements, leaving anomalies only in the non-vaporizing refractory elements.

The Omega Nebula, a star and solar system nursery, is some 5,500 light-years away. The lumpy features in the dense cold gas and dust are illuminated by stars off the upper left of the image and may themselves represent sites of future star formation. Colors in the fog of surrounding hotter material indicate the nebula’s chemical make up. The predominately green glow corresponds to abundant hydrogen, with trace sulfur and oxygen atoms contributing red and blue hues. The picture spans about 3 light-years. {NASA, ESA, J. Hester (ASU)}

I asked Nie about the importance of the findings that nucleosynthetic anomalies of volatile elements had been found in meteorites from our solar nebula, and the resulting conclusion that the solar nebula was likely not homogeneous as previously accepted but rather was mixed and heterogeneous.

She answered that “understanding the chemical composition of the solar nebula and its heterogeneity is a fundamental question.”

She said that if the solar nebula is relatively homogeneous, then most likely the building blocks of planets (e.g., planetesimals) would largely be made of the same or similar compositions. But if the solar nebula is largely heterogeneous, then large isotopic variations in the make-up of planetesimals should be considered when modeling how the planets grow.

“The two scenarios (homogeneous or heterogeneous) would probably result in very different isotopic compositions of hypothetical planets when planetesimals are assembled in a certain way,” she wrote in an email.

“In turn, if we want to reproduce the observed isotopic compositions of planets, the two scenarios will likely require different ways of assembling planetesimals….When we think about planetary accretion and formation, any proposed models have to be consistent with the chemical and isotopic constraints,” meaning that the models have to take into account new findings about the nature of the solar nebula.

This all matters because, as Nie put it, “early solar system processes that formed planets are still poorly understood.”

There are, for instance, many competing theories on how planets grow — the theory of “pebble accretion” versus collisions by increasingly massive planetesimals, or maybe a mixture of both — and on such a basic question as how water (the key volatile) arrived on Earth.

Illustration of a dusty disk in orbit around a young star. In the pebble accretion theory, rocky matter from the size of centimeters to meters in diameter collect to form planetesimals in a protoplanetary disk.  As Nie explained, the paper’s findings suggest problems with the pebble accretion theory that pebbles travel freely from the outer solar system to the inner solar system.  She said the potassium isotopes show a clear separation between outer and inner solar systems reservoirs and very limited amounts of outer solar system material in the Earth. Thus, she said, pebbles must be largely isolated from each other by a barrier such as Jupiter. (Wikipedia)

Anat Shahar, a co-author on the Science paper and a staff scientist at the Earth and Planets Lab of the Carnegie Institution of Science, also thinks the paper is significant because it gives evidence that volatile carriers can survive the formation of our solar system and that the materials that made our planets are not uniform.