This week in astronomy news: Inflated helium atmospheres surround two exoplanets, and the Kepler Space Telescope captures the moments around a supernova that hint at a companion star triggering the explosion.

Helium puffs up some exoplanet atmospheres

Illustration of helium escaping exoplanet HAT-P-11b
Artist’s impression of the exoplanet HAT–P–11b with its extended helium atmosphere blown away by the star, an orange dwarf star smaller, but more active, than the Sun.
Denis Bajram / Univ. of Geneva</em

Like a couple of day-old party balloons, two planets in our galaxy appear to be puffed up with helium and leaking it into space.

Helium is the second most abundant element in the universe after hydrogen. Despite that, researchers have only recently started detecting it in the atmospheres of exoplanets.

The hope is that helium, being the second lightest element, could be a sensitive probe of how sunlight erodes planetary skies. Astronomers suspect that bloated gas giants snuggled up to their stars might lose most of their atmospheres due to the relentless influx of high-energy ultraviolet and x-ray radiation.

Now, two worlds are granting researchers their wish. Lisa Nortmann (University of La Laguna, Spain) and colleagues, using the 3.5-meter telescope at Calar Alto Observatory in Spain, detected a trail of helium dragging behind the Saturn-mass exoplanet WASP–69b (located about 160 light-years away) as it orbits its sun about once every 4 days. The comet-like helium tail extends roughly 170,000 kilometers (110,000 miles) away from the planet.

In a companion paper relying on data from the same telescope, Romain Allart (University of Geneva, Switzerland) and colleagues found a puffed-up helium atmosphere — five times as wide as the planet itself — enveloping the roughly Neptune-mass world HAT–P–11b, which orbits a star about 120 light-years away. Spectra reveal that helium winds appear to blow from the dayside to the nightside at nearly 10,000 kilometers per hour (7,000 mph).

These two finds, both published in the December 7th Science, are not the first time that researchers have detected helium around an exoplanet. That honor goes to the planet WASP–107b — astronomers reported the detection of helium escaping that world in May. These more recent reports, however, are the first time that astronomers have been able to resolve details about the helium atmospheres.

Supernova body-slams a companion

Illustration of white dwarf siphoning gas off a companion star
This artist's conception shows a white dwarf (left) siphoning material from a larger star, a process that will eventually cause a stellar explosion.

The potential explanations for what makes a star go boom are many and varied, and astronomers debate the exact underlying causes. Now, a detailed timeline of a supernova at the moment it burst on to the scene is providing new details to aid researchers in this quest.

The late Kepler Space Telescope (may it rest in peace) is best known for its thousands of exoplanet discoveries. But in its waning years, it became a useful tool for providing snapshots every 30 minutes of rapidly evolving phenomena in the cosmos. In February, it caught the moments before, during, and after the detection of a supernova, dubbed 2018oh, that went off in a galaxy roughly 160 million light-years away.

Right away, the build up of light seemed odd. In the first five days following the explosion, the supernova seemed to emit an excess of light compared to other similar supernovae. After that, its evolution proceeded pretty much as expected.

Reporting November 25th on the astronomy preprint site, Georgios Dimitriadis (University of California, Santa Cruz) and colleagues suspect that the excess light came from the supernova blast wave slamming into a companion star — a companion that may have triggered the detonation.

Supernova 2018oh is classified as a Type 1a supernova, which means it likely arose from the destruction of a white dwarf, the naked core left behind after stars like our Sun run out of fuel. There are two leading ideas for what would cause a white dwarf to explode. One possibility is that two white dwarfs locked in a gravitational embrace slam together. Another scenario is that a run-of-the-mill companion star keeps dumping gas on to the white dwarf until it becomes too heavy to support its own weight, triggering a thermonuclear explosion.

The double white dwarf story seems to explain most Type 1a supernovae, according to a 2014 review paper. But a handful, such as 2018oh, appear to be caused by a single white dwarf pilfering gas from a neighbor. Dimitriadis’ team notes that a few scenarios provide a good match to the observations — including the presence of a thin layer of helium enveloping the doomed star or the presence of an off-kilter nugget of nickel in its core — but a lone white dwarf appears slightly favorable.


L. Nortmann et al. “Ground-Based Detection of an Extended Helium Atmosphere in the Saturn-Mass Exoplanet WASP-69b.” Science. December 7, 2018.

R. Allart et al. “Spectrally Resolved Helium Absorption from the Extended Atmosphere of a Warm Neptune-Mass Exoplanet." Science. December 7, 2018.

J. Spake et al. “Helium in the Eroding Atmosphere of an Exoplanet.” Nature. May 2, 2018.

G. Dimitriadis et al. “K2 Observations of SN 2018oh Reveal a Two-Component Rising Light Curve for a Type Ia Supernova.” November 25, 2018.

D. Maoz, F. Mannucci, and G. Nelemans. “Observational Clues to the Progenitors of Type Ia Supernovae.” Annual Review of Astronomy & Astrophysics. August 2014.


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