By studying the magnetic orientations in ancient meteorites, an MIT team has determined that the solar nebula — the vast of disc of gas and dust that ultimately gave rise to the solar system — lasted around 3 to 4 million years. Image: NASA/JHUAPL
Study finds the swirling gas disk disappeared within the solar system’s first 4 million years
About
4.6 billion years ago, an enormous cloud of hydrogen gas and dust
collapsed under its own weight, eventually flattening into a disk called
the solar nebula. Most of this interstellar material contracted at the
disk’s center to form the sun, and part of the solar nebula’s remaining
gas and dust condensed to form the planets and the rest of our solar
system.
Now scientists from MIT and their colleagues have estimated the
lifetime of the solar nebula — a key stage during which much of the
solar system evolution took shape.
This new estimate suggests that the gas giants Jupiter and Saturn
must have formed within the first 4 million years of the solar system’s
formation. Furthermore, they must have completed gas-driven migration of
their orbital positions by this time.
“So much happens right at the beginning of the solar system’s
history,” says Benjamin Weiss, professor of earth, atmospheric, and
planetary sciences at MIT. “Of course the planets evolve after that, but
the large-scale structure of the solar system was essentially
established in the first 4 million years.”
Weiss and MIT postdoc Huapei Wang, the first author of this study, report their results today in the journal Science.
Their co-authors are Brynna Downey, Clement Suavet, and Roger Fu from
MIT; Xue-Ning Bai of the Harvard-Smithsonian Center for Astrophysics;
Jun Wang and Jiajun Wang of Brookhaven National Laboratory; and Maria
Zucolotto of the National Museum in Rio de Janeiro.
Spectacular recorders
By studying the magnetic orientations in pristine samples of ancient
meteorites that formed 4.653 billion years ago, the team determined that
the solar nebula lasted around 3 to 4 million years. This is a more
precise figure than previous estimates, which placed the solar nebula’s
lifetime at somewhere between 1 and 10 million years.
The team came to its conclusion after carefully analyzing angrites,
which are some of the oldest and most pristine of planetary rocks.
Angrites are igneous rocks, many of which are thought to have erupted
onto the surface of asteroids very early in the solar system’s history
and then quickly cooled, freezing their original properties — including
their composition and paleomagnetic signals — in place.
Scientists view angrites as exceptional recorders of the early solar
system, particularly as the rocks also contain high amounts of uranium,
which they can use to precisely determine their age.
“Angrites are really spectacular,” Weiss says. “Many of them look
like what might be erupting on Hawaii, but they cooled on a very early
planetesimal.”
Weiss and his colleagues analyzed four angrites that fell to Earth at different places and times.
“One fell in Argentina, and was discovered when a farm worker was
tilling his field,” Weiss says. “It looked like an Indian artifact or
bowl, and the landowner kept it by this house for about 20 years, until
he finally decided to have it analyzed, and it turned out to be a really
rare meteorite.”
The other three meteorites were discovered in Brazil, Antarctica, and
the Sahara Desert. All four meteorites were remarkably well-preserved,
having undergone no additional heating or major compositional changes
since they originally formed.
Measuring tiny compasses
The team obtained samples from all four meteorites. By measuring the
ratio of uranium to lead in each sample, previous studies had determined
that the three oldest formed around 4.653 billion years ago. The
researchers then measured the rocks’ remnant magnetization using a
precision magnetometer in the MIT Paleomagnetism Laboratory.
“Electrons are little compass needles, and if you align a bunch of
them in a rock, the rock becomes magnetized,” Weiss explains. “Once
they’re aligned, which can happen when a rock cools in the presence of a
magnetic field, then they stay that way. That’s what we use as records
of ancient magnetic fields.”
When they placed the angrites in the magnetometer, the researchers
observed very little remnant magnetization, indicating there was very
little magnetic field present when the angrites formed.
The team went a step further and tried to reconstruct the magnetic
field that would have produced the rocks’ alignments, or lack thereof.
To do so, they heated the samples up, then cooled them down again in a
laboratory-controlled magnetic field.
“We can keep lowering the lab field and can reproduce what’s in the
sample,” Weiss says. “We find only very weak lab fields are allowed,
given how little remnant magnetization is in these three angrites.”
Specifically, the team found that the angrites’ remnant magnetization
could have been produced by an extremely weak magnetic field of no more
than 0.6 microteslas, 4.653 billion years ago, or, about 4 million
years after the start of the solar system.
In 2014, Weiss’ group analyzed other ancient meteorites that formed
within the solar system’s first 2 to 3 million years, and found evidence
of a magnetic field that was about 10-100 times stronger — about 5-50
microtesla.
“It’s predicted that once the magnetic field drops by a factor of
10-100 in the inner solar system, which we’ve now shown, the solar
nebula goes away really quickly, within 100,000 years,” Weiss says. “So
even if the solar nebula hadn’t disappeared by 4 million years, it was
basically on its way out.”
The planets align
The researchers’ new estimate is much more precise than previous estimates, which were based on observations of faraway stars.
“What’s more, the angrites’ paleomagnetism constrains the lifetime of
our own solar nebula, while astronomical observations obviously measure
other faraway solar systems,” Wang adds. “Since the solar nebula
lifetime critically affects the final positions of Jupiter and Saturn,
it also affects the later formation of the Earth, our home, as well as
the formation of other terrestrial planets.”
Now that the scientists have a better idea of how long the solar
nebula persisted, they can also narrow in on how giant planets such as
Jupiter and Saturn formed. Giant planets are mostly made of gas and ice,
and there are two prevailing hypotheses for how all this material came
together as a planet. One suggests that giant planets formed from the
gravitational collapse of condensing gas, like the sun did. The other
suggests they arose in a two-stage process called core accretion, in
which bits of material smashed and fused together to form bigger rocky,
icy bodies. Once these bodies were massive enough, they could have
created a gravitational force that attracted huge amounts of gas to
ultimately form a giant planet.
According to previous predictions, giant planets that form through
gravitational collapse of gas should complete their general formation
within 100,000 years. Core accretion, in contrast, is typically thought
to take much longer, on the order of 1 to several million years. Weiss
says that if the solar nebula was around in the first 4 million years of
solar system formation, this would give support to the core accretion
scenario, which is generally favored among scientists.
“The gas giants must have formed by 4 million years after the
formation of the solar system,” Weiss says. “Planets were moving all
over the place, in and out over large distances, and all this motion is
thought to have been driven by gravitational forces from the gas. We’re
saying all this happened in the first 4 million years.”
This research was supported, in part, by NASA and a generous gift from Thomas J. Peterson, Jr.
Jennifer Chu | MIT News Office
