Astronomy news this past week includes findings on the water content of Trappist-1's seven Earth-size planets, a new measure of the solar system's planetary formation, and an explanation for luminous, short-lived supernovae.
Trappist-1 Planets May Be Water-rich, Hint at Migration
The seven Earth-size planets orbiting Trappist-1, a dim star not much larger than Jupiter, may contain far more water than previously thought, according to results appearing March 19th in Nature Astronomy.
Observations had already shown that the planets are not dense enough to be pure rock and thick atmospheres had also been ruled out. A lighter element, probably water in ice, liquid, or vapor form, must be involved in the lightweight planets’ makeup.
Cayman Unterborn (Arizona State University) and colleagues use a computer code to combine information about the entire Trappist-1 system, including the planets’ radii and the star’s chemical makeup. The code suggested that the inner two planets (b and c) have less than 15% water by mass, while the outer planets (f and g) are more than half water by mass. (Previous results based solely on the planets’ masses and radii had found considerably lower water masses.)
The results suggest that the outer planets formed outside of the “frost line,” where water in the protoplanetary disk existed in ice form and would have been more easily accumulated, then migrated to their current close-in orbits. Read more in the Arizona State University press release.
Researchers Measure Timeline of Planetary Formation
In the March 22nd Nature, Martin Schiller (University of Copenhagen, Denmark) and colleagues have pieced together a history of the solar system — using calcium.
The team focused on the abundances of two isotopes, calcium-48 and calcium-44, measuring the ratio on Earth, in meteorites known as angrites and ureilites, which come from two distinct parent bodies, and in meteorites from Mars, the Moon, and Vesta.
The researchers found that the ratio grew with the parent body mass. The trend suggests all planetary bodies in the solar system’s protoplanetary disk of dust, gas, and pebbles, grew at the same rate — smaller bodies just stopped growing earlier. The new view on planet formation has interesting implications. For example, a longstanding Moon formation scenario has a Mars-sized impactor colliding with a fully formed Earth; this new formation scenario instead implies two half-Earth-sized masses collided to make the Earth-Moon system.
Some details and discrepancies remain to be worked out, but “the authors’ work adds a missing piece to the jigsaw puzzle of planet formation,” commented Alessandro Morbidelli (Observatory of Côte d’Azur, France).
How Fast Can Stars Die?
Researchers report in the March 26th Nature Astronomy the discovery of a luminous supernova that vanished within a month, designated KSN 2015K. The discovery, using the Kepler space telescope, is the most extreme example so far of a class of exploding stars dubbed fast-evolving luminous transients (FELTs), which peak quickly and fade fast.
Armin Rest (Space Telescope Science Institute) and colleagues comb through possible explanations of KSN 2015K’s light curve, discarding most. For example, a typical explanation for a supernova’s light is the radioactive afterglow of unstable elements created in or shortly before the explosion. However, this radiation lasts too long to explain these observations.
Instead, the team thinks that a dense cocoon of material, ejected from the star shortly before the blast, initially hid the supernova’s radiation. Only when the outer layers of the star break through the cocoon could we witness the light. However, questions remain about what kind of progenitor star would have shed its outer layers shortly before its demise.