Some say there is nothing that beats the joy of watching a child grow up. But what about watching a planet grow up?
Looking into a nebula, like the Orion Nebula pictured above, is like peering through the window of a nursery. In this stunning cloud of cosmic dust, you can spot stars and planets in the early stages of their development, their fusion-filled centers swaddled in rotating disks of gas and space dust. It is these “protoplanetary” disks around newborn stars that ultimately condense into the stars’ orbiting planets. Voila! A Solar System is born!
Editor's note: Protoplanetary disks do not just occur in nebulas, but they are a great place to look for them.
Look at all those baby stars and their protoplanetary disks! NASA/ESA and L. Ricci (ESO)
Scientists at EPL and elsewhere are working hard to elucidate the steps that bring a rocky planet like ours into the Universe. Just as the same basic genetic and metabolic processes happen in all humans but result in a vast variety of appearances and preferences, the same physical processes at work in disks result in a variety of types of planets. Among these planets are super-Earths unlike anything in our Solar System, planets with crazy orbits, and at least one lovely 3rd rock from the Sun that supports life.
We can’t watch a planet grow up in real-time. That would take millions if not billions of years, and humans only get about 100 on a good run. Instead, we peer into space and piece together a puzzle of clues that we observe in nature and test in the lab.
Learning about planet formation requires scientists to find and observe many developing planets and star systems at once. Then, we compare and contrast them. In this article, we will share five ways that the scientists at EPL are studying the birth and development of planets.
Editor’s note: This is not a comprehensive list. Links to even more research at the bottom!
An artist’s impression of protoplanets forming around a young star, courtesy of NRAO/AUI/NSF; S. Dagnello
To study the various phases of planet development, it helps to be able to find the planets! Since the first planet orbiting a Sun-like star outside of our Solar System was confirmed in 1995 by EPL’s Paul Butler, scientists have discovered more than 4,000 “exoplanets,” but only a handful of them are still in their formative years.
To catch a planet while it’s still growing up, you need to find it when it is only a few million years old. For comparison, our planet is about 4.5 billion years old.
In 2018, a research team that included Carnegie astronomer Jaehan Bae, developed a new planet finding technique. The team measured carbon monoxide, which absorbs a very distinct wavelength of light from the central star. Small changes in the wavelength indicate changes in the movement of the gas in the protoplanetary disk.
The unique movement of gas that they found is a pretty good sign that young planets are orbiting the star! In this case, they found two Jupiter-sized baby planets orbiting HD 163296, a young 4 million-year-old star that is twice the size of our Sun in the Sagittarius constellation.
At the time, Jaehan Bae stated, “Looking ahead, analyzing the movement of material in a disk around a young star could help us find exoplanets while they are still in their most-formative stages,” Bae concluded. “This could really help us understand how the architecture of a planetary system comes to be and maybe even unlock mysteries about the evolution of our own Solar System.”
LaPaz Icefield 02342 seen here in thin section under polarized light courtesy of Carles Moyano-Cambero.
Comets and asteroids represent what is left over from our Solar System’s formation. They are essentially the chunks of rock, ice, and metal that didn’t make it into planets, so finding a piece of one can be like unlocking a time capsule from that tumultuous stage of our Solar System’s growth. Fortunately, bits of asteroids fall to the Earth as meteorites. By studying their composition and mineralogy, researchers like Carnegie’s Larry Nittler can uncover details about our Solar System’s formation!
For example, the LaPaz meteorite is a “carbonaceous chondrite” that was discovered in Antarctica’s LaPaz Icefield. Meteorites like LaPaz probably formed somewhere near Jupiter. Meanwhile, comets and other icy bodies are thought to have formed far out past Neptune.
So when Nittler discovered a tiny grain that had the same fingerprint as a comet inside of LaPaz, it was like finding a bug stuck in amber—except the bug is the history of our Solar System. The fact that the comet piece was inside of this meteorite supports the idea that materials moved from the outer edges of our Solar System inward while the planets we know and love were forming.
According to Nittler, “Because this sample of cometary building block material was swallowed by an asteroid and preserved inside this meteorite, it was protected from the ravages of entering Earth’s atmosphere,” Nittler explained. “It gave us a peek at material that would not have survived to reach our planet’s surface on its own, helping us to understand the early Solar System’s chemistry.”
Editors note: This is a simplification of meteors, comets, and asteroids. You can learn more about these three objects here.
Artist's impression of the surface of the exoplanet Barnard's Star b courtesy of ESO/M.Kornmesser
Whether a planet can grow up to sustain life depends on what’s going on beneath its surface.
Carnegie’s Anat Shahar, Peter Driscoll, Alycia Weinberger, and George Cody argue that a true picture of planetary habitability must consider how a planet’s atmosphere is linked to and shaped by what’s happening in its interior.
On Earth, plate tectonics are crucial for maintaining a surface climate where life can thrive. As tectonic plates move and interact across the surface of our planet some plates sink under other plates. The sinking of the plates below the planet’s surface drives the cycling of materials between Earth’s surface and interior. This circulation is the motor that effectively removes heat from Earth’s interior, which ultimately drives the Earth’s magnetic field created by convective motions in Earth’s molten iron outer core. Without a magnetic field, Earth would be bombarded by solar and cosmic radiation. Not good for life on the surface!
“We need a better understanding of how a planet’s composition and interior influence its habitability, starting with Earth,” Shahar said. “This can be used to guide the search for exoplanets and star systems where life could thrive, signatures of which could be detected by telescopes.”
A scanning electron microscope image of one of the experiments in Elardo and Shahar's paper that shows a bright, semi-spherical metal (representing a core) next to a gray, quenched silicate (representing a magma ocean). Credit: Stephen Elardo.
Scientists can’t take samples of planets’ cores. But they can make mini-planets in the lab to study iron chemistry under high pressures!
Within tens of millions of years of Earth formation, its core formed through a process called differentiation—when the denser materials, like iron, sank inward toward the center. This formed the layered composition we know today, with an iron core and a silicate mantle and crust.
One key to understanding this period of Earth’s differentiation is studying and comparing the variation in the distribution of iron isotopes in samples of ancient rocks and minerals from Earth, the Moon, and other planets or planetary bodies. Basically, isotopes of an element have different numbers of neutrons. More neutrons make an isotope heavier than the atomic weight you might see on a periodic table. Having more neutrons can also change the way the element behaves in chemical reactions, at least very slightly. The isotopic composition thus can act as a fingerprint for the origin of a certain material.
An outstanding mystery has been the difference in iron isotope ratios found in lavas generated by melting Earth’s upper mantle (thanks volcanoes!) and samples from primitive meteorites, asteroids, the Moon, and Mars. The iron isotopic composition of lavas produced from our mantle is on average heavier than the found on the Moon and elsewhere in the Solar System. If everything in the Solar System started out from the same protoplanetary disk—which it did—the iron isotopic composition should look pretty much the same from planet to planet. So, why is Earth’s iron isotopic composition different ?
Using state of the art high-pressure tools, Carnegie’s Stephen Elardo and Anat Shahar were able to mimic the conditions found deep inside Earth and other planetary bodies to determine why iron isotopic ratios vary so much.
The scientists were able to create tiny versions of these planetary bodies and subject them to the conditions under which they are thought to have formed. It turned out the answer was the presence of another element: nickel.
Under the conditions in which the Moon and Mars formed, the presence of nickel makes the mantle hold on to high concentrations of lighter iron isotopes. However, under the hotter and higher-pressure conditions of Earth’s core formation process, this nickel effect disappears. This helps explain why lavas from Earth have generally heavier iron isotopes than other planetary bodies.
Rick Carlson sitting on 4.28 billion-year-old crust on the shores of Hudson's Bay. Photo credit Jonathan O'Neil.
Earth is around 4.5 billion years old today, but once upon a time, our planet was young—only a few million years old. At that time, the Earth’s surface was likely a seething magma ocean. Early on, the planet’s dense iron and nickel sunk to its interior to form the core. And as the surface of the planet cooled by releasing heat to outer space, bits of magma began to solidify and our planet’s crust began to form, floating on top of the magma.
Something similar probably happened on most of our Solar System’s terrestrial worlds, including our Moon, which likely formed after a collision with Earth in the late stages of its development. But there is one huge difference. The Moon’s crust is composed predominantly of just a single mineral and the majority of the lunar crust is over 4.3 billion years old. Only the big impact craters on the Moon are filled in by younger lava flows. The reason the lunar crust is so dominated by a single mineral, called plagioclase feldspar, is because plagioclase is less dense than the magma it crystallizes from, so as plagioclase crystallized it floated to the top of the magma ocean for the same reason that ice floats on water.
In contrast, Earth’s crust is composed of a number of different rock types and is uniquely characterized among the rocky planets in having two types of crust, the magnesium-rich crust of the ocean basins and the silica-rich (think granite) crust of the continents. The crust is very young, at least compared to that on the Moon. Three-quarters of Earth’s crust, that beneath the oceans, is 200 million years old or younger. Only a few extremely small areas of the continental crust are more than 3.8 billion years old. This provides an interesting challenge to Rick Carlson, Director of the Earth and Planets Laboratory, who seeks out Earth’s oldest rocks in order to understand the nature of our planet’s first crust.
The prevailing theory, which Carlson's study challenges, suggests that the Earth probably formed a static crust like our terrestrial neighbors. Then something happened to make it change into the unique, dynamic planet we have now.
Since he can’t get a sample of Earth’s first crust, Carlson investigated this idea using a new form of radiometric dating. Unlike traditional radiometric dating that uses long-lived radioactive isotopes like uranium, this new technique uses short-lived radioactive isotopes like samarium and tungsten to explore specifically the nature of the crust that existed in the first 50-500 million years of Earth’s history.
What he found was surprising. Based on the isotopic signatures of the decay products of these elements in some of Earth’s oldest rocks, Carlson determined that Earth’s first crust was probably the same type of crust we have now in the ocean basins – a rock type called basalt, like that erupted in Hawaii.
Carlson explained, “If you think about the magma ocean on the Moon forming a crust, it's basically a one-time event. The Earth must also have formed some type of early crust, which we don’t have samples of, but appears to have gotten rid of it right away through crust subduction, much like occurs today via plate tectonics. Taking water-rich basaltic crust into the high temperatures of Earth’s interior causes it to melt, and those melts are similar in composition to the silica-rich rocks typical of continental crust. Granitic rocks are sufficiently buoyant that they can survive at Earth’s surface for billions of years, so we can still find remnants of this small fraction of the old crust, the oldest of which discovered so far dates back to 4.37 billion years.
Carlson concluded, “Earth has been producing and recycling crust basically the same way throughout its history and remains geologically active likely because of the two-way exchange of material between surface and interior.”
Carnegie's Anat Shahar Presents a Brief History of Planet Formation
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