Astronomers have discovered that planets may be different from the swirling envelopes of gas and dust from which they were born. The revelation appears to suggest that scientists’ favored models of planet formation may be too simplistic.
A team led by researchers at Northwestern University in Illinois made the discovery while observing a still-forming planet and the disk of birth material in which it resides.
this exoplanet The core of this research is PDS 70bone gas giant planet Approximately three times the mass and width Jupiter It is 369 light years away from us. PDS 70b orbits the star about 20 times faster distance between earth and sunit takes 119.2 Earth years to complete one week.
Just as we expect children to look like their parents, scientists also expect planets to bear similarities to the disks of material surrounding baby stars, called protoplanetary diskfrom which they are formed, and then occupy them in the course of evolution.
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“The widely accepted picture of planet formation may be too simplistic for observational astrophysicists,” team leader Chih-Chun “Dino” Hsu of Northwestern University said in a statement. “According to this simplified picture, The ratio of carbon to oxygen in a planet’s atmosphere should match the ratio of carbon to oxygen in its birth disk—assuming the planet accreted material through the gas in its disk. Instead, we found a planet with less carbon than the disk. The proportion of oxygen is much lower.
“Now, we can confirm our suspicion that the planet formation process is an oversimplification.”
Planet kids go their own way
Planets are born when material around a newborn star forms a protoplanetary disk. Over millions of years, small amounts of cold gas and dust in this disk came together under its own gravity to form the seeds of planets.
This phase of the investigation planetary evolution Until recently, progress was hampered by the fact that we couldn’t directly observe exoplanets inside the envelopes in which they were born. Generally speaking, by the time planets become visible, they have aged and their envelopes have dissipated.
However, the system surrounding the newborn star PDS 70 is different. There, two planets, PDS 70b and PDS 70c, can be distinguished from their envelopes, with an estimated age of about 5 million years.
“In this system, we see two planets still forming, as well as the materials from which they formed,” said team member Jason Wang, an assistant professor of physics and astronomy at Northwestern University. “Previous studies have analyzed this gas disk to understand its composition.
“For the first time we were able to measure its composition planets still forming itself, to see how similar the material on the planet is compared to the material in the disk.
To determine the chemical composition of the disk and PDS 70b, the team examined light from the young star system. This research is possible because chemical elements absorb and emit characteristic wavelengths of light. This means that chemicals in the atmosphere leave “fingerprints” on the starlight that passes through them.
The research team developed new techniques that make faint features visible that would otherwise be indistinguishable.
“These new tools make it possible to take really detailed spectra of faint objects next to really bright objects,” Wang said. “Because the challenge here is having a very faint planet next to a real planet. bright stars. It’s difficult to isolate a planet’s light to analyze its atmosphere.
From this data, the team was able to obtain information about the water and carbon monoxide surrounding PDS 70b. With this information, they can infer the ratio of carbon to oxygen in the planet’s atmosphere. The team then compared this to the proportions of the same gas in the disks from which the planets formed.
“We initially expected Carbon-oxygen ratio The carbon content in the planet may be similar to that in the disk,” Xu said. “But, instead, we found that the ratio of carbon to oxygen in the planet was much lower than in the disk. This was somewhat surprising and suggests that our widely accepted picture of planet formation is oversimplified.
The team doesn’t know exactly what causes PDS 70b to deviate from its prenatal cloud, but they have two possible explanations.
The first one will see the PDS 70b form before Protoplanetary disks are rich in carbon. Alternatively, the exoplanet could have grown by devouring large amounts of solid material rather than just gas.
“If the Earth preferentially absorbs ice and dust, then the ice and dust would evaporate before entering the Earth,” Wang said. “So, it may tell us that we can’t just compare gas to gas. The solid component may have an impact on the carbon-oxygen ratio A big impact.”
The next step for the researchers is to examine the star system PDS 70 more closely. This will include studying the planet PDS 70c, a companion star to PDS 70b.
“By studying these two planets together, we can better understand the system’s formation history,” Xu said. “However, this is just one system. Ideally, we need to identify more systems to better understand how planets form.”
The team’s findings were published Wednesday (December 18) in the Astrophysical Journal Letters.