Scientists searching for water on three distant exoplanets – planets outside our solar system – 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 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 planets outside the solar system,” said study co-author Peter McCullough, of the Space Telescope Science Institute in Baltimore.
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.
To study the planets’ water content, the researchers observed each planet while it passed in front of its star. 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 as dim as they had suspected. The planet had a little water, but not enough to significantly lower the star’s light.
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.