From laboratory experiments to observations of young star systems, University of Hawai‘i (UH) at Mānoa researchers are on a quest to understand how rocky planets like Earth form.
Planets form from disks of gas and dust that surround young stars: previous research has shown that nearly all stars are born with such disks, and revealed hints of planet formation within them. Surveys for planets around other stars, termed “exoplanets,” have discovered that Earth-size and presumably rocky planets are common, and many stars have planets orbiting much closer to their host star than the Earth-Sun distance. But most of the steps between dust and planets are poorly understood, in part because they are obscured within the inner region of these proto-planetary disks.
The National Science Foundation (NSF) and National Aeronautics and Space Administration (NASA) recently awarded a total of $1.3 million in three separate grants to teams of UH Mānoa scientists from the Department of Earth Sciences and Hawaii Institute of Geophysics and Planetology (HIGP) in the School of Ocean and Earth Science and Technology (SOEST), the Institute for Astronomy, and the Information and Computer Science Department (ICS) to explore this inner realm around other stars — and our Sun — in search of the secrets to planet formation.
SOEST Earth Sciences professor Eric Gaidos, lead investigator on two of the grants, explained, “the story of planet formation is like an epic movie, where we could watch only the dramatic opening scene and the happy ending, but missed everything between, leaving us guessing about the main characters, their roles and most of the plot.”
Because of the vast distances to even the nearest young stars, the most powerful telescopes are usually unable to resolve details in the inner zones of planet-forming disks. Instead investigators must employ indirect means to probe these regions. Gaidos and his collaborators are using instances where dusty material from the disk briefly blocks part of the star, causing hours-long dimming or “dipping” in the starlight.
While the phenomenon was discovered decades ago using telescopes on the ground, space telescopes such as NASA’s Spitzer, Kepler and TESS spacecraft have revealed that the “dipper” phenomenon is common around young stars, and could be produced by a combination of structures in a disk or dust-producing bodies like asteroids or comets that could be the building-blocks of planets. Much smaller amounts of dust can be detected much closer to a star in this way and the properties of the dust and any accompanying gas can be determined by analyzing the scattering and absorption of starlight at different wavelengths. These observations can be readily obtained from the ground, but the rotation of Earth means that a single telescope cannot continuously monitor a star.
The centerpiece of the NSF-funded effort is an observing program using the Las Cumbres Observatory, a global network of small robotic telescopes that can continuously monitor a star, handing off the task from telescope to telescope as the Sun rises on one site and sets at another. This network also includes a set of smaller cameras with telephoto lenses called the All-Sky Automated Survey for Supernovae (ASAS-SN), which scans most of the sky each night.
The team, including co-investigators Ben Shappee at the Institute for Astronomy, Peter Sadowski of the ICS, and IfA graduate student Suchitra Narayanan, is developing improved algorithms to precisely measure changes in the brightness of stars and to detect dimming events in near-real time to direct observations by other telescopes.
A second effort, supported by NASA and involving IfA graduate student Alexa Anderson, re-purposes a satellite called Swift that usually looks for the flashes of high-energy radiation coming from distant cosmic explosions to monitor these “dipper” stars instead. Its instruments are sensitive to ultraviolet and X-ray radiation that is absorbed by Earth’s atmosphere and cannot be observed from the ground. Observations at these wavelengths will indicate whether there is gas—in addition to dust—that is in orbit around these stars. The amount of gas, primarily hydrogen and helium, compared to dust is an important aspect of planet formation models.
Only two planets—Mercury and Venus—orbit at a distance from the Sun within the range of exoplanets detected by the NASA Kepler mission. Both are poorly explored neighbors of Earth, but both may hold important secrets to understanding the formation of rocky planets, including planets that are Earth-like and potentially habitable.
The third award, also from NASA, will support an effort led by HIGP researcher Bin Chen, joined by Gaidos, to understand the origin and properties of Mercury’s metal iron core. Mercury is anomalous in the relatively large size of its core and the chemistry of its surface—both suggesting that the planet formed under much more oxygen-poor conditions than Earth.
Using laboratory experiments that reproduce the pressure and temperature found in the interior of Mercury and computer models, Chen and Gaidos will investigate how this enigmatic planet could have formed and how its different chemistry affected the fate of light-weight elements such as carbon early in the planet’s history. In Earth’s silicate mantle, carbon is typically in an oxidized form and emerges as carbon dioxide when molten rock erupts as lava to the surface, e.g. through volcanoes. In the interior of Mercury, carbon might be in the form of iron-carbon alloys (analogous to steel), graphite or even diamond, dissolving into the core or appearing in the planet’s surface crust. This research will guide studies of the many small exoplanets found on close-in orbits around their stars and that may resemble Mercury.
“The magnetic field in rocky planets such as Earth and Mercury offers clues concerning their internal structures and dynamics. To decipher how Mercury’s magnetic field is generated within its core, we can build the planet from scratch with analog ingredients through laboratory experiments. We are trying to answer how the planet’s initial conditions and early processes would affect the composition of its core and mantle, surface chemistry and mineralogy, and internal dynamics such as the generation of a magnetic field.” said Chen.
The research uses special “anvils” made of sintered diamonds to squeeze samples with a pressure half a million times our atmosphere. Graduate student Keng-Hsien Chao is performing these experiments in Chen’s Multi-Anvil Press Laboratory (MAPLab) and the Advanced Photon Source of the Argonne National Laboratory in Illinois.
“The first golden age of planetary exploration began almost 60 years ago with space probes to other planets in the Solar System,” said Gaidos. “The second began 30 years later with the discovery of planets around other stars. We are on the threshold of a third age when these two scientific adventures coalesce, and we better understand the origin of Earth and other rocky planets in a cosmic context.”