Planetary scientists searching for water on three distant exoplanets have come up dry. Scientists who trained NASA’s Hubble Space Telescope on three “hot Jupiters” have discovered that they have far less water vapor than previously thought.
The findings, published in the Astrophysical Journal Letters, show that current theories on how such planets form and migrate may not hold much water -- much like the planets themselves.
“This is just a baby step in measuring the composition of other plants outside the solar system,” said study co-author Peter McCullough, a planetary scientist at the Space Telescope Science Institute in Baltimore. “We don’t even really understand the Earth -- and we live on it.”
Using the Hubble Space Telescope, the scientists examined the atmospheres of three gas-giant planets orbiting searingly close to their respective home stars: HD 189733b, HD 209458b and WASP-12b, which sit between 60 and 900 light-years from Earth.
Even though hot Jupiters don’t host life-friendly environments, they’re great for searching for water. With surface temperatures between 1,500 and 4,000 degrees Fahrenheit, the planet’s water ends up in the atmosphere as vapor, making it easier for space telescopes to observe. (Ironically, we still don’t know how much water is in our own nearby gas giant, Jupiter, McCullough said -- it’s so cold that all the water sinks down into the planet, far out of sight.)
To study the planets’ water content, the researchers observed each planet while it passed in front of its star, blocking some of its star’s light. Some starlight also passes through the planet’s atmosphere, where water molecules absorb certain wavelengths.
In wavelengths of light where water leaves its fingerprint, the star looks dimmer. In wavelengths where water molecules don’t absorb light, the star shouldn’t look as dim.
Sure enough, the star was indeed dimmer in the water-absorbing wavelengths than it was in other parts of the light spectrum -- but it wasn’t nearly as dim as they had suspected. The planet had a little water, but not enough to significantly lower the star's light.
The water abundance was on the order of a tenth to a thousandth of the level they had expected, given current models of planet formation.
"The very low water vapor levels we measure challenge our understanding of the chemistry involved in planet formation," study co-author Nicolas Crouzet of the University of Toronto said in an email.
So does this mean there’s less water in alien planets all around? Probably not, McCullough said -- the abundance on hot gas giants doesn’t necessarily have much to do with the water abundance on small rocky planets. It’s an apples-to-oranges comparison.
Björn Benneke, a Caltech planetary scientist who did not work on the paper, said it’s also possible that the water content was obscured by haze or clouds on the hot Jupiters’ surfaces.
But Benneke is working with McCullough and others to use the same technique on super-Earths, to learn more about the atmospheres of this strange class of planets, which have no analogy in our own solar system.
“I think the super-Earths will be more exciting because they are [among] the most frequent planets in the galaxy but we have no examples in the solar system,” Benneke said. Studying these planets will be more difficult, because they’re not as big as the gas giants, he said. But they could help to reveal whether these strange, massive planets are more like the gas giants, or more like smaller, rocky planets -- the kind that could potentially host life as we know it.
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