Local Hubble engineer makes sense of how scientists detect distance objects in space
Sometimes, it’s very handy to have a spaceflight optical engineer in one’s contact list.
In the Thursday, Sept. 12, edition of the Times Observer, there was a wire story announcing the discovery that there is water in the atmosphere of a planet far outside our solar system.
That planet, K2-18b, orbits a red dwarf star 110 light years away.
That’s about 647 trillion miles.
For comparison, our sun is about 93 million miles away. The light from the sun reaches us in a little over eight minutes. Voyager 1 was launched in 1977, travels at 38,000 miles an hour, and is a little short of 14 billion miles from earth. It is headed toward a star that is much closer to Earth — about 17.6 light years — than is K2-18b. Voyager’s estimated time of arrival is 40,000 years.
K2-18b is far away.
Telescopes can pick up the light from its star well enough that scientists can tell when something — a planet — cuts between that star and the telescope on a regular basis.
To determine that there is water in the atmosphere takes a little bit more than noticing a periodic dimming of the light from the star.
Information from that Hubble Space Telescope was used in making that determination.
To begin to understand how we can know something like that at a range of more than half-a-quadrillion miles, the Times Observer reached out to Optical Engineer John Mangus, of Warren, who has worked with NASA on Hubble from before it was launched through its servicing missions. He has been involved in the development of the James Webb Space Telescope, as well.
“Most stars emit electromagnetic radiation given by a well-defined equation called Planck’s Law,” Mangus said.
The wavelength — color — of the light tells us the temperature of the source.
Scientists know what our sun — or a distant red dwarf — should look like based on the wavelengths of the light it is giving off.
They also know that we, standing on Earth, do not see the sun’s light as it truly is.
Clouds block a good portion of the light from the sun, of course. Otherwise invisible particles block sunlight, too. The sun looks different at noon than it does at sunset, because the light is traveling through less atmosphere at noon.
“There are many dips in the spectra because of absorption by molecules in our atmosphere,” Mangus said. “This is just one reason why we place telescopes on tall mountains — to get above much of our atmosphere.”
K2-18b’s star is cooler than our sun and appears red. Scientists like Mangus know what the light from K2-18b’s star should look like.
“The object you asked about, K2-18b, is eight times the size of Earth and orbiting around a red dwarf,” Mangus said. “So, if we observe K2-18b when it is in between our line of sight to the red dwarf, the light from the red dwarf must pass through K2-18b’s atmosphere.”
“Any deviations in the red dwarf’s spectrum… must be due to the molecules in K2-18b’s atmosphere,” he said. “This is called transit spectroscopy.”
If the image we had of that red dwarf were collected on Earth, there would be more atmosphere — Earth’s — to consider. But, “we can ignore the effects of Earth’s atmosphere because the spectrum was measured by the widefield camera in Hubble Space Telescope which is far above us — 335 miles — in the vacuum of space,” Mangus said.
Knowing what changes water and other molecules make to the wavelengths of light isn’t enough, according to Mangus. How fast the star is moving away from us also changes the apparent wavelengths.
That is another thing that the two teams of scientists who announced the discovery had to take into account, in coming to the conclusion that there is indeed some amount of water in the atmosphere of exoplanet K2-18b that is 110 light years away.