A Kepler-438b Caveat (and a Digression)

Before we go interstellar, a digression with reference to yesterday’s post, which looked at how we manipulate image data to draw out the maximum amount of information. I had mentioned the image widely regarded as the first photograph, Joseph Nicéphore Niépce’s ‘View from the Window at Le Gras.’ Centauri Dreams regular William Alschuler pointed out that this image is in fact a classic example of what I’m talking about. For without serious manipulation, it’s impossible to make out what you’re seeing. Have a look at the original and compare it to the image in yesterday’s post, which has been processed to reveal the underlying scene.

Image: New official image of the first photograph in 2003, minus any manual retouching. Joseph Nicéphore Niépce’s View from the Window at Le Gras. c. 1826. Gernsheim Collection Harry Ransom Center / University of Texas at Austin. Photo by J. Paul Getty Museum.

And here again is the processed image, a much richer experience.

The University of Texas offers this explanation of how the image was made:

“Niépce thought to capture this image using a light-sensitive material so that the light itself would “etch” the picture for him. In 1826, through a process of trial and error, he finally came upon the combination of bitumen of Judea (a form of asphalt) spread over a pewter plate. When he let this petroleum-based substance sit in a camera obscura for eight hours without interruption, the light gradually hardened the bitumen where it hit, thus creating a rudimentary photo. He “developed” this picture by washing away the unhardened bitumen with lavender water, revealing an image of the rooftops and trees visible from his studio window. Niépce had successfully made the world’s first photograph.”

As with many astronomical photographs, what the unassisted human eye would see is often the least interesting aspect of the story. While we always want to know what a person looking out a window would see, we learn a great deal more by subjecting images to a variety of filters.

Meanwhile, in the Rest of the Galaxy…

Habitable zone planets are a primary attraction of the exoplanet hunt, but so often a tight analysis shows that what we know of a world isn’t enough to confirm its habitable status. Kepler-438b is a case in point, a world that is likely rocky orbiting a red dwarf some 470 light years away in the constellation Lyra. The planet orbits the primary every 35.2 days, but writing in these pages last January, Andrew LePage estimated there was only a one in four chance that Kepler-438b is in the habitable zone, declaring it more likely to be a cooler version of Venus.

Now we have more evidence that a planet some in the media have called ‘Earth-like’ is in fact a wasteland, its chances of life devastated by hard radiation from the host star. Kepler-438 produces huge flares every few hundred days, each of them approximately ten times more powerful than anything we’ve ever recorded on the Sun. These ‘superflares’ are laden with energies of 1033 erg, although energies of 1036 erg have been observed.

But the flares are part of a larger problem for Kepler-438b. They are associated with coronal mass ejections (CMEs), a phenomenon likely to have stripped away the planet’s atmosphere entirely. In work to be published in Monthly Notices of the Royal Astronomical Society, David Armstrong (University of Warwick, UK) and colleagues analyze conditions around the red dwarf. Armstrong explains in a University of Warwick news release:

“If the planet, Kepler-438b, has a magnetic field like the Earth, it may be shielded from some of the effects. However, if it does not, or the flares are strong enough, it could have lost its atmosphere, be irradiated by extra dangerous radiation and be a much harsher place for life to exist.”

Image: The planet Kepler-438b is shown here in front of its violent parent star. It is regularly irradiated by huge flares of radiation, which could render the planet uninhabitable. Here the planet’s atmosphere is shown being stripped away. Credit: Mark A Garlick / University of Warwick.

The relationship of flares and CMEs is complicated, as are the effects of a magnetic field. From the paper:

It is possible that CMEs occur on other stars that produce very energetic flares, which could have serious consequences for any close-in exoplanets without a magnetic field to deflect the influx of energetic charged particles. Since the habitable zone for M dwarfs is relatively close in to the star, any exoplanets could be expected to be partially or completely tidally locked. This would limit the intrinsic magnetic moments of the planet, meaning that any magnetosphere would likely be small. Khodachenko et al. (2007) found that for an M dwarf, the stellar wind combined with CMEs could push the magnetosphere of an Earth-like exoplanet in the habitable zone within its atmosphere, resulting in erosion of the atmosphere. Following on from this, Lammer et al. (2007) concluded that habitable exoplanets orbiting active M dwarfs would need to be larger and more massive than Earth, so that the planet could generate a stronger magnetic field and the increased gravitational pull would help prevent atmospheric loss.

A coronal mass ejection occurs when huge amounts of plasma are blown outward from the star, and the extensive flare activity on Kepler-438 makes CMEs that much more likely. With the atmosphere greatly compromised or stripped away entirely, the flares can do their work, bathing the surface in ultraviolet and X-ray radiation and a sleet of hard particles. For a time, Kepler-438b looked so intriguing from an astrobiological standpoint, especially with its small radius 1.1 the size of Earth’s, but it takes an optimistic assessment of the habitable zone indeed to include it in the first place, and it now appears that the chances for life here are remote.

The paper is Armstrong et al., “The Host Stars of Keplers Habitable Exoplanets: Superflares, Rotation and Activity,” accepted at MNRAS and available as a preprint.