Prevailing theories about planet formation are contradicted by new data
"Nature is fundamentally trying to tell us something that we haven’t considered in our planetary models."
In 2014, scientists found one of the key components of life out in the cosmos — an exoplanet with water vapor in its atmosphere. We now know that this was no anomaly. Most exoplanets likely have water vapor in their atmospheres, but just how much water is difficult to detect.
Now, a team of scientists has discovered that while most exoplanets do indeed have water vapor in their atmospheres, the amount is far less than expected. The data instead reveal trends that upend established theories of how planetary systems form — including our own Solar System.
The findings are the culmination of five years of work by Nikku Madhusudhan, a Cambridge University scientist — beginning with that first observation made in 2014. They add fuel to the fire of a heated debate over how gas giants like Jupiter formed and provide new data that could help further the search for water-abundant planets in the cosmos.
Madhusudhan and his team pulled data from the Hubble Space Telescope, Spitzer Space Telescope, the Very Large Telescope in Chile and the Gran Telescopio Canarias in Spain. They focused in on 19 exoplanets, which ranged in size from a ‘mini-Neptune,’ or a mass of about 10 Earths, to a ‘super Jupiter,’ or a mass equivalent to more than 600 Earths. Using transit spectroscopy, they found water vapor in the atmosphere of 14 planets. They also found sodium in six planets’ atmospheres, and potassium in another six.
But when they looked at the abundance of water, the scientists were in for a shock: Water was significantly depleted in every one of the exoplanets’ atmospheres.
The findings were published this week in the Astrophysical Journal Letters.
Water, water everywhere — but not enough
The observations contradict current models of planet formation. For years, scientists predicted that the elements in a planet and its host star would match up — assuming they formed out of the same material. And based on what we know about the chemistry of the planets in our solar system, the prediction was that not only would that be the case, but the elements would be overabundant in the planets.
"The amount of oxygen we had expected these planets to accrete in the form of water ice was optimistic."
On that basis, the prevailing theory held that of all the elements that would be enhanced in planets’ atmospheres, oxygen would be the most obvious, Madhusudhan tells Inverse. That’s because of all the solid materials planets form from, water ice is the most dominant. Gas giants’ atmospheres are rich in hydrogen, so the oxygen would be held as H2O — water.
“You should see plentiful water in all these atmospheres. But what we are seeing in reality is that the water is significantly depleted across the board in all the planets that we are seeing,” he says.
The fact that the finding held across the “broad census” of planets is also surprising, Madhusudhan says. That suggests this is a consistent trend across exoplanets, not some outlier data.
“I maintain the view that nature is fundamentally trying to tell us something that we haven’t considered in our planetary models,” he says. “Maybe in many of these planets, when they are forming, they are not accreting as much solids as we thought they were.”
Current ideas of planetary formation suggest that planets form from a proto-planetary disc of material that swirls around their star. The larger the planet, the more solids it is likely to accrete while forming. The more solids, the more water ice. But that might not be the case.
“One hypothesis is that maybe they are forming far out in the proto-planetary disc, and far enough that all the ices have condensed out and the gas is very poor in oxygen,” Madhusudhan says. If the planet doesn’t remain in the proto-planetary disc for long, then it wouldn’t have the time to re-accrete the lost oxygen, resulting in an oxygen-poor atmosphere.
“The bottom line is that the amount of oxygen we had expected these planets to accrete in the form of water ice was optimistic.”
The data hold profound implications for how the planets in our own Solar System formed, too.
How scarce is water in the Solar System?
While the new findings upend prevailing assumptions of planetary chemistry and how they form, these assumptions are based on incomplete data. Scientists can only infer so much from the observations we have.
“We don’t know the oxygen abundance in the Solar System,” Madhusudhan says. Part of the reason for that is that it is hard to measure water vapor in the atmosphere of extremely cold planets, like Jupiter and the other gas giants. These exoplanets, by contrast, are all quite hot — some as hot as 4,500 degrees Fahrenheit.
"We should find planets that are overabundant."
If a planet’s atmosphere is very cold, then the water — and the oxygen — will have condensed, making it hard to spot, Madhusudhan says. Instead, scientists have based the idea that oxygen should be so abundant on the fact that other elements seem to be equally enhanced.
These data don’t fully contradict that theory — the scientists found that sodium and potassium were abundant as expected in the exoplanets’ atmospheres. But they do introduce the concept that not all elements may be equal — meaning you can’t infer that oxygen — or any other element — will be equally enhanced as any other.
More answers should be available soon — NASA’s Juno mission is actively looking for water in the atmosphere of Jupiter.
“If Juno finds that the oxygen is indeed as high as expected for Jupiter, that would tell us that these exoplanets formed differently from the Solar System,” Madhusudhan says. Conversely, if it finds water is depleted, then “that is even more interesting,” he says.
Larger samples will also help settle this heated debate, he says — the findings apply only to these gas giants. Whether water abundance is as expected on rocky planets more similar to Earth is difficult to predict, he says. “But I wouldn’t be surprised if we find water below our expectations.”
“We should inevitably find planets that are not under-abundant, we should find planets that are overabundant,” he says. The data will speak for itself — regardless of what Juno finds on Jupiter, “either way we stand to learn something very fundamental.”
“That is the fun of science at the very cutting edge, you can see these pillars of knowledge being shaken once in a while and exoplanets is just that sort of subject,” Madhusudhan says. “Every so often we see our assumptions about the Solar System and planetary systems in general are being ruthlessly tested by nature again and again, and this is just one more point in that saga.”