G 9-40b: Confirming a Planet Candidate


M-class dwarfs within 100 light years are highly sought after objects these days, given that any transiting worlds around such stars will present unusually useful opportunities for atmospheric analysis. That’s because these stars are small, allowing large transit depth — in other words, a great deal of the star’s light is blocked by the planet. Studying a star’s light as it filters through a planetary atmosphere — transmission spectroscopy — can tell us much about the chemical constituents involved. We’ll soon extend that with space-based direct imaging.

While the discoveries we’re making today are exciting in their own right, bear in mind that we’re also building the catalog of objects that next generation ground telescopes (the extremely large, or ELT, instruments on the way) and their space-based cousins can examine in far greater depth. And it’s also true that we are tuning up our methods for making sure that our planet candidates are real and not products of data contamination.

Thus a planet called G 9-40b orbiting its red dwarf host about 90 light years out is significant not so much for the planet itself but for the methods used to confirm it. Probably the size of Neptune or somewhat smaller, G 9-40b is a world first noted by Kepler (in its K2 phase) as the candidate planet made transits of the star every six days. Confirmation that this is an actual planet has been achieved through three instruments. The first is the Habitable-zone Planet Finder (HPF), a spectrograph developed at Penn State that has been installed on the 10m Hobby-Eberly Telescope at McDonald Observatory in Texas.

HPF provides high precision Doppler readings in the infrared, allowing astronomers to exclude possible signals that might have mimicked a transiting world — we now know that G 9-40b is not a close stellar or substellar binary companion. HPF is distinguished by its spectral calibration using a laser frequency comb built by scientists at the National Institute of Standards and Technology and the University of Colorado. The instrument was able to achieve high precision in its radial velocity study of this planet while also observing the world’s transits across the star.

A post on the Habitable Zone Planet Finder blog notes that the brightness of the host star (given its proximity) and the large transit depth of the planet makes G 9-40b “…one of the most favorable sub-Neptune-sized planets orbiting an M-dwarf for transmission spectroscopy with the James Webb Space Telescope (JWST) in the future…”

But the thing to note about this work is the collaborative nature of the validation process, putting different techniques into play. High contrast adaptive optics imaging at Lick Observatory showed no stellar companions near the target, helping researchers confirm that the transits detected in the K2 mission were indeed coming from the star G 9-40. The Apache Point observations using high-precision diffuser-assisted photometry (see the blog entry for details on this technique) produced a transit plot that agreed with the K2 observations and allowed the team to tighten the timing of the transit. The Apache Point observations grew out of lead author Guðmundur Stefánsson’s doctoral work at Penn State. Says Stefánsson:

“G 9-40b is amongst the top twenty closest transiting planets known, which makes this discovery really exciting. Further, due to its large transit depth, G 9-40b is an excellent candidate exoplanet to study its atmospheric composition with future space telescopes.”

Image: Drawn from the HPF blog. Caption: Precise radial velocities from HPF (left) on the 10m Hobby-Eberly Telescope (right) allowed us to place an upper limit on the mass of the planet of 12 Earth masses. We hope to get a further precise mass constraint by continuing to observe G 9-40 in the future. Image credit: Figure 11a from the paper (left), Gudmundur Stefansson (right).

Near-infrared radial velocities from HPF allowed the 12 MEarth mass determination, the tightening of which through future work will allow the composition of the planet to be constrained. All of this is by way of feeding a space-based instrument like the James Webb Space Telescope with the data it will need to study the planet’s atmosphere. In such ways do we pool the results of our instruments, with HPF continuing its survey of the nearest low-mass stars in search of other planets in the Sun’s immediate neighborhood.

The paper is Stefansson et al., “A Sub-Neptune-sized Planet Transiting the M2.5 Dwarf G 9-40: Validation with the Habitable-zone Planet Finder,” Astronomical Journal Vol. 159, No. 3 (12 February 2020). Abstract / preprint.


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