A shot in the dark

Gregory Nemec

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Hundreds of light-years away, a planet much like Jupiter rides an orbit so close to its sun that gasified iron, magnesium, and chromium float and swirl through its atmosphere. The planet is called MASCARA-2 b, and it is very (very) hot—though in the evening there may be enough of a chill, relatively speaking, for traces of iron to condense from the air, just as the soft dew of our own lovely planet condenses in evening’s shadow.

It was a remarkable feat—seeing this planet, peering so far with such clarity—and it was accomplished by the EXtreme PREcision Spectrometer, or EXPRES, an instrument built at Yale and installed on a telescope near Flagstaff, Arizona. Researchers determined the atmospheric composition of MASCARA-2 b using “transit spectroscopy.”

“You can use this method when a planet passes in front of its star along our line of sight,” says Sam Cabot, a graduate student in the Department of Astronomy who led the data analysis. A planet blocks light from the star behind it (as when Mercury or Venus passes between Earth and the sun). But the thin halo of a planet’s atmosphere may also block certain colors of starlight. Iron, magnesium, and chromium, for example, each block a unique set of colors. The spectrometer compares the star’s full spectrum with the colors that make it through the planet’s atmosphere, providing a long-distance elemental analysis of that atmosphere.

Looking at the makeup of alien atmospheres is a secondary capability of EXPRES. It was designed by Yale astronomy professor Debra Fischer and her team in order to find exoplanets, or planets beyond our solar system. It is in this realm that its “extreme precision” truly shines: it measures the infinitesimal oscillations of far-burning stars and, from these oscillations, infers whether a planet is orbiting that star—tugging it back and forth with its own small gravitational pull.

The process of exoplanet discovery requires months, even years, of observational data collection. And so, says Cabot, “in the interim, we might do a bit more atmospheric characterization”—more nights staring into the void, calling to mysterious planets, asking what they’re made of.