Discovered: Youngest Fully-Formed Exoplanet Ever

MAUNAKEA, Hawaii – A team of Caltech-led researchers discovered the youngest fully-formed exoplanet ever detected using the W. M. Keck Observatory on Maunakea, Hawaii and the Kepler Space Telescope. The planet, K2-33b, at five to 10 million years old, is still in its infancy. A paper detailing the finding appears this week in the journal Nature.

Planet formation is a complex and tumultuous process that remains shrouded in mystery. Astronomers have discovered roughly 2,000 planets orbiting stars other than our Sun — however, nearly all are middle-aged, with ages of a billion years or more. For astronomers, attempting to understand the life cycles of planetary systems using existing examples is like trying to learn how people grow from babies to children to teenagers, by only studying adults.

The first signals of the planet’s existence were measured by NASA’s Kepler space telescope during its K2 mission. The telescope detected a periodic dimming in the light emitted by the planet’s host star, K2-33, that hinted at the existence of an orbiting planet. 

Observations from the W. M. Keck Observatory in Hawaii validated that the dimming was indeed caused by a planet, using both of the 10 meter Keck telescopes: the HIRES instrument installed on Keck I and NIRC2 on the Keck II.

“HIRES was used to measure the Doppler shift (radial velocity) of the star over time, and confirm that the orbiting companion is a planet,” said Trevor David, first author on the paper and a graduate student working with professor of astronomy Lynne Hillenbrand. “The high resolution spectra were also used to confirm the youth of the star, measure its temperature and how fast it is rotating, and rule out the presence of any additional stars in the system.”

High resolution images obtained with Keck Observatory’s NIRC2 instrument in 2011 and 2016 were also used to confirm that there were no other nearby stars (either gravitationally bound to K2-33, or in the background but aligned by chance) that might be mimicking a planet transit signal.

By comparison, the new discovery is a new born. “At 4.5 billion years old, the Earth is a middle-aged planet — about 45 in human-years,” David said. “The planet K2-33b would be an infant of only a few weeks old.”

“This discovery is a remarkable milestone in exoplanet science,” said Erik Petigura, a postdoctoral scholar in planetary science and a coauthor on the paper. “The newborn planet K2-33b will help us understand how planets form, which is important for understanding the processes that led to the formation of the earth and eventually the origin of life.”

When stars form, they are encircled by dense regions of gas and dust called protoplanetary disks, from which planets form. By the time a young star is a few million years old, this disk has largely dissipated and planet formation is mostly complete.

The star orbited by K2-33b has a small amount of disk material left, indicated by observations from NASA’s Spitzer space telescope, indicating that it is in the final stages of dissipating.

“Astronomers know star formation has just completed in this region, called Upper Scorpius, because roughly a quarter of the stars still have bright protoplanetary disks,” David said. “The remainder of stars in the region do not have such disks, so we reasoned that planet formation must be nearly complete for these stars, and there would be a good chance of finding young exoplanets around them.”

K2-33b, like many other exoplanets, was detected due to the periodic dimming in the central star’s light as the planet passes in front of it. By studying the frequency of dips in the star’s light and measuring by how much the light dimmed, the team was able to determine the size and orbital period of the planet. K2-33b is “a remarkable world,” Petigura said. The exoplanet, which is about six times the size of Earth, or about 50 percent larger than Neptune, makes a complete orbit around its host star in about five days. This implies that it is 20 times closer to its star than Earth is to the Sun.

K2-33b is a large planet like the gas giants in our solar system. In our solar system these giant planets are all far from the Sun. As it turns out, the proximity of the giant planet K2-33b to its star is not too out of the ordinary for planets in our galaxy — many have been discovered close in, often completing an orbit around their parent star in weeks or even days. The explanation for this is that large planets can be formed far from their star and migrate inward over time. The position of K2-33b so close to its parent star at such an early age implies that if migration occurred, it must have occurred quickly. Alternatively, the planet could be evidence against the migration theory, suggesting that giant planets can in fact form close in to their stars.

K2-33b is fully formed, but it may still evolve over time. The next step is to measure the planet’s mass and determine its density. These measurements will offer insights into the planet’s fate later in life—whether it will stay roughly the same size or if it will cool and contract.

“In the last 20 years, we have learned that nature can produce a staggering diversity of planets—from planets that orbit two stars to planets that complete a full orbit every few hours,” Petigura said. “We have much to learn, and K2-33b is giving us new clues.”

The findings are detailed in a paper titled, “A Neptune-sized transiting planet closely orbiting a 5-10 million-year old star.” The work was supported by a National Science Foundation Graduate Research Fellowship and included data funded by NASA. Professor Lynne Hillenbrand, staff scientist David Ciardi, and senior faculty associate in astronomy Charles Beichman were additional Caltech coauthors on this paper.

The W. M. Keck Observatory operates the largest, most scientifically productive telescopes on Earth. The two, 10-meter optical/infrared telescopes near the summit of Maunakea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrographs and world-leading laser guide star adaptive optics systems.

HIRES (the High-Resolution Echelle Spectrometer) produces spectra of single objects at very high spectral resolution, yet covering a wide wavelength range. It does this by separating the light into many “stripes” of spectra stacked across a mosaic of three large CCD detectors. HIRES is famous for finding planets orbiting other stars. Astronomers also use HIRES to study distant galaxies and quasars, finding clues to the Big Bang.

NIRC2 (the Near-Infrared Camera, second generation) works in combination with the Keck II adaptive optics system to obtain very sharp images at near-infrared wavelengths, achieving spatial resolutions comparable to or better than those achieved by the Hubble Space Telescope at optical wavelengths. NIRC2 is probably best known for helping to provide definitive proof of a central massive black hole at the center of our galaxy. Astronomers also use NIRC2 to map surface features of solar system bodies, detect planets orbiting other stars, and study detailed morphology of distant galaxies.

Keck Observatory is a private 501(c) 3 non-profit organization and a scientific partnership of the California Institute of Technology, the University of California and NASA.