Image Courtesy of Breanna Brownson.
Imagine if Earth’s orbit shrunk to 1.5 percent of its current radius. The Sun would swallow the sky, temperatures would soar to devastating heights, and Earth’s surface would be completely consumed by oceans of lava. A year, or a full rotation around the Sun, would pass in 17.5 hours—but you wouldn’t see its completion, as you’d never survive on the scorching surface at a temperature of two thousand degrees Celsius. This world, a so-called “hell planet,” exists forty light years from Earth, but its fiery exterior and apocalyptic atmosphere are not what make this burning alien world interesting.
This planet, formally called 55 Cancri e, has an ultra-short period orbit, meaning that it essentially hugs its star as a full revolution takes under eighteen hours. The planet is known as a “super-Earth” since it is just larger than our home planet—except Earth orbits the Sun from a safe, habitable distance of ninety-three million miles rather than a shocking 1.4 million miles. Many extrasolar planets, or exoplanets, which orbit in close proximity to their stars are hot Jupiters: large, Jupiter-like planets that take under ten days to complete their orbits. Being so large and so close to their stars makes detection easy; the real challenge lies in detecting the smaller, Earth-like planets whose measurements are often drowned out by the noise from the stars they orbit.
Armed with the EXtreme PREcision Spectrograph (EXPRES), an ultra-precise instrument that can make these difficult measurements, Yale astronomy researchers Debra Fischer and Andrew Szymkowiak are chasing otherwise elusive low-mass planets like 55 Cancri e. At first, astronomers thought the orbit was four times larger, as the blink-and-you’ll-miss-it nature of the orbit evaded proper study. But Harvard graduate student Rebekah Dawson correctly interpreted its signal, prompting recent Yale PhD graduate Lily Zhao to spearhead efforts to accurately characterize 55 Cancri e. By studying smaller planets of varying alignments and orbital distances, researchers can better understand how planetary systems form. But our current understanding is biased by our measurements of majority-large planets. Large, high-mass planets like those similar to Jupiter are easier to detect, so we have more data characterizing them, which is why we know the least about the smaller planets that make up the majority of planetary systems.
EXPRESsing Precise Data
EXPRES was developed by Fischer at Yale and installed at the Lowell Discovery Telescope at the Lowell Observatory in Flagstaff, Arizona. It uses the Doppler effect to detect planets based on the motion of their stars. As a planet orbits a star, it exerts a small gravitational effect. The star moves very slightly in response, shifting the frequency of its measured light. Scientists measure that Doppler shift using spectrographs, instruments that separate stars’ light into their component spectra. When the wavelengths of the spectra are bluer than expected, meaning they have shorter wavelengths, the star is being tugged toward us. Redder, or longer, wavelengths indicate movement away from us. These Doppler shifts can be described as changes in a star’s radial velocity, the measured motion of the star away from or toward an observer. Over time, radial velocity measurements can be used to determine properties of orbiting planets that are essential to their characterization, such as mass, orbital period, and distance from the star.
Other instruments have detected thousands of exoplanets by the starlight they block as they pass between the star and its observer, but the resulting dimming from these transits only yields the radii of each planet. With the radial velocity method, researchers can also calculate how much it weighs. Using both mass and radius measurements allows them to determine the planets’ densities. From this parameter, researchers can infer composition—in short, is it an ice planet or a rocky planet?
However, radial velocity methods may produce data with large uncertainties due to instrumental interference, atmospheric effects, and intrinsic stellar variability, which could smother the near-imperceptible gravitational stellar effects caused by low-mass planets. EXPRES sought unprecedented radial velocity precision of a ten-centimeter-per-second Doppler shift, which is the motion that Earth induces on the sun as it orbits.
When the project was first proposed, Fischer faced doubt from many prominent voices in the astronomy community, as they believed the stars’ large velocities would mask those of small planets. But she persevered. Today, EXPRES is able to detect Earth-sized planets at a precision of twenty to thirty centimeters per second, and other spectrographs have followed. “I think the performance of EXPRES emboldened the community to think, ‘Maybe we can get this precision,’ and it’s definitely worth doing,” Fischer said.
And its precision will only keep improving. Newer spectrographs on bigger telescopes may push the precision to five to ten centimeters per second, and upcoming projects involving the James Webb Space Telescope and Habitable Worlds Observatory may provide the data to characterize hundreds more Earth-like planets. “They’ll be able to block out the light of the star, see the planets sitting around the star, and collect spectra of the atmospheres of those little pale blue dots. So this field will look back someday and laugh at how crude everything is right now,” Fischer said.
55 Cancri e: A Hell of a Planet
55 Cancri’s large, gaseous planets were among the first discovered outside our solar system, supporting the existence of multi-planet systems. Its large signal attracted astronomers as the study of exoplanets emerged in the late nineties. Using EXPRES data, Yale astronomers could detect and characterize the smaller planets in the system, uncovering more about planets with ultra-short orbital periods and the formation of their planetary systems, as EXPRES’ higher precision is especially valuable for understanding planetary architectures, or structures, of multi-planet systems.
Knowing that Earth induces a ten-centimeter-per-second shift on the Sun as it orbits, the EXPRES team modeled a tiny signal of just forty centimeters per second. The velocity signal is a function of the spin and angle of the star and indicates that 55 Cancri e is a small planet that orbits its star along its equator. This signal would have been lost on most other spectrographs, highlighting the necessity of EXPRES’ precision in the search for low-mass planets such as super-Earths.
55 Cancri e’s ultra-close orbit defies traditional models of planetary formation. Astronomers believe that small rocky planets form inside the ‘snowline’ of protoplanetary disks, which is the region within the dense gas surrounding a newly formed star (like the Sun) that is located near Mars in our Solar System. However, the current location of 55 Cancri e is thought to be too hot for even rocky planet formation. Furthermore, its orbit doesn’t match the other known planets in the system. These clues suggest that the planet formed in a farther, cooler orbit and somehow migrated inward, altering its orbit as it neared the star’s equator.
So how did it migrate in? Did 55 Cancri e gently spiral in and then find a parking spot relatively close to the star? Did it get gravitationally kicked in by other planets? Planet migration is a hotly debated topic in astronomical communities, and little is known. But the observation that this planet’s orbital plane is aligned with its stellar equator is consistent with a more gentle inward migration—which could occur as other material, dust, and gas exert a slow, dragging force on the planet—as opposed to a quick gravitational interaction, bringing researchers one step closer to understanding planetary migration and architectures.
55 Cancri is particularly interesting because researchers already understand much about it, such as its five, tightly-packed orbiting planets, and now they are beginning to understand how planets in the system may have migrated. “Once we understand that as a sort of general principle, [that] it’s true that planetary systems are dynamically packed, then we can start to extrapolate about what that means for all of the worlds around the four hundred billion stars in the Milky Way galaxy. And then, the probability of life,” Fischer said.