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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.

{ 10 comments… add one }
  • Steven Janowiecki February 26, 2020, 13:49

    Hi there. It looks like you grabbed the wrong figure/caption pairing from the HPF blog post – you’re showing the graph/photo from Gummi’s Figure 6 with the caption from Figure 4. It jumped out to me because that is not a graph of a transit (it’s an RV curve) and not a picture from APO (it’s HET).

    • Paul Gilster February 26, 2020, 14:09

      Absolutely right, and thanks for catching that. I’ve swapped in the correct caption. Much appreciated.

  • Enzo February 26, 2020, 16:19

    There’s an interesting talk here :

    about building very low cost ($50-100M) telescopes with resolving power of 25 m, specifically for imaging exoplanets.

    • Ashley Baldwin February 27, 2020, 14:34

      Probably a bit more than $50-100 million . This is the basis to the Colossus 60m scope and its smaller cousin the ELF telescope – et al . They are essentially made up of multiple small off axis telescopes combining into a “partially filled “aperture interferometer. The idea has been around for some years now but doesn’t seem to have progressed beyond concept . This always make me wary – implying a significant technological leap is required to win over the big funding required from the NSF and Universities .( the NSF has proven surprisingly and notoriously reluctant to support ground based scopes like the LSST and GMT) .

      The telescopes in this lecture are reliant on a breakthough in mirror glass, to allow creation of uniquely thin and light , metre wide segments. These in turn would require the much less robust and heavy mountings seen in conceptual images of such scopes. As opposed to similarly sized “conventional” ELTs.

      The current biggest obstacle to ELT – or better still, OWL telescopes , is mass . Especially of the mount. The relation, if I recall, is that building costs increase with the square of the finished telescope mass. By that measure , a 50-60m Keck Style ELT would come in at eye watering approaching $5 billion ( though ironically much smaller than the JWST , and unlike it – serviceable, upgradable and with a much longer life span) . That’s five times the cost of the E-ELT and TMT.

      Breaking the mass/cost equation is one of the great challenges facing modern telescope designers and has led to the concepts described here by Jeff Kuhn from the UoH (who has posted on this site previously ) . I hope he is reading to pass comment on what is an exciting idea. Too date only the Hobby-Eberly telescope – used with this discovery – and its copy, the SALT- have achieved this via novel construction, materials and design. But at the price of acting only as “light buckets” only for spectrographic analysis and with limited sky coverage and tracking. With no imaging. .

      Nothing seems to gave moved forward with Colossus et al recently so I would guess there is still work to be done maturing its mirror glass to a technological level .

      Lots of concepts in astrophysics and related instrumentation are inadvertently misrepresented as being more mature than in reality . A recent example would be coronagraphs for exoplanet imaging. The sort of one billion times contrast reductions required for imaging planets round Sun like stars were being described routinely in what were effectively hypothetical devices over ten years ago. Forcoronagraphs that have only just caught up and become reality over the last few years . Thanks in no small way to the outstanding and dedicated pre-formulation work done for the WFIRST telescope.

  • Harry R Ray February 27, 2020, 10:51

    K2-18 b may be HABITABLE AFTER ALL!!!!! “The interior and atmosphere of the habitable zone exoplanet K2-18b.” by Nikku Madhusudhan, Matthew C Nixon, Luis Wellbanks, Anjali A A Piette, Richard A Booth. K2-18 b’s H/HE envelope mass fraction may be less than 1/100,000th the TOTAL mass of K2- 18 b, allowing for a surface in the LIQUID PHASE! Although life as we know it cannot ORIGINATE on K2-18 b. if panspermia is real, WHO KNOWS?

    • Harry R Ray February 27, 2020, 17:24

      REALITY CHECK: Even the ABSOLUTE BEST CASE SCENARIO would mean atmospheric pressures at the SURFACE of a liquid water ocean comparable or greater by a factor of ten to the water pressure at the bottom of Challenger Deep! Life exists THERE, but the real problem would be that the atmosphere is SO DENSE that ABSOLUTLY NO STARLIGHT would ever reach the surface. ABSOLUTELY PITCH BLACK! Awaiting Andrew Le Page’s Exoplanet Habitability Reality Check on the revised parameters for this exoplanet.

  • Ronald February 27, 2020, 11:46
    • Harry R Ray February 28, 2020, 10:37

      The main question that MUST be answered here is: Why are the methane and ammonia levels so depleted from the expected norms? Is this endemic in ALL habitable zone sub-neptunes like K2-18 b and G9-40 b, and if so, could they ALL have liquid water surfaces. Could this depletion be due to BIOLOGICAL PROCESSES? I assume that scope time has been reserved on HST to search for water vapor and possible depleted methane and ammonia on G9-40b, too.

  • Michael Fidler February 28, 2020, 2:22

    Some interesting information on this system, the M2.5 red dwarf is 9.9 billion years old. This should make for very evolved planet and if it is tidally locked, high ocean levels at solar zenith and on the opposite hemisphere. If it has a large moon like earth’s it could be as large as Mars but what effect would the large oceans have on the orbit of such a moon? We are getting into a territory that has no comparison in our solar system so expect some interesting surprises.

    • Harry R Ray February 29, 2020, 10:41

      Michael Fidler: “Some interesting information on this system…” Which system are you commenting on, the G9-40 system or the K2-18 system?

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