The Von Trapp Family Planets: A Sixth Planet Confirmed for K2-138

Title: K2-138 g: Spitzer Spots a Sixth Planet for the Citizen Science System

Authors: Kevin Okay. Hardegree-Ullman, Jessie L. Christiansen, David R. Ciardi, Ian J. M. Crossfield, Courtney D. Dressing, John H. Livingston, Kathryn Volk, Eric Agol, Thomas Barclay, Geert Barentsen, Björn Benneke, Varoujan Gorjian, Martti H. Kristiansen

First Writer’s Establishment: Caltech / IPAC-NExScI

Standing: Accepted for publication in AJ, out there on arXiv

What do you get when you might have five exoplanets that sing and add a sixth? Why, the K2-138 system in fact! 

Found in 2018 by means of the Exoplanet Explorers program, K2-138 was the primary system discovered by citizen scientists with K2, the extension mission of the unique Kepler program. By recognizing common dips in K2 mild curves, the citizen scientists have been capable of finding 4 sub-Neptune exoplanets, with a further super-Earth found after further analysis. All of the planets have been discovered to be in a close to good 3:2 resonance chain, which means their orbital durations observe successive ratios of one another – as mentioned on this Astrobite.

However, the K2-138 system had extra to supply! The evaluation which found the super-Earth additionally noticed two further dips within the K2 mild curves, roughly 42 days aside. Dips like these, proven by the darkish blue traces, and the letter g, in Determine 1, point out {that a} sixth planet would possibly transit K2-138, ready to be confirmed by the authors of at this time’s paper.

A two panel figure showing the light curves of K2-138 over a period of 80 days. The top panel shows the raw flux of the star K2-138. The flux is represented by a solid black line which varies semi-periodically wtih varying amplitudes, peaking at 100.5% and 99% of the star's normal relative flux. Small colored lines extend vertically from the black line to show the transits of the planets and occur at the same period of each planet. The lines are typically 0.1% long. From shortest period to longest, planet b is shown with red lines, planet c with orange lines, planet d with yellow lines, planet e with green, planet f with light blue and the potential planet g with dark blue. Directly beneath this, the bottom panel shows the flux of K2-138 flattened. The black line is now constant at 100% relative flux across the 80 day width of the graph. The transit depth lines are still shown in the same colours as before, but are also now marked with the letter of each planet. The lines now extend between 0.1% for planet b, to 1.2% for planet e, with the other planets at depths in between. Grey circles are scattered around the lines showing the individual flux measurements.

Determine 1: The highest panel exhibits the uncooked K2 mild curve, whereas the underside panel exhibits the identical mild curve flattened to spotlight the planetary transits. In each panels transits of every planet within the system are proven with coloured traces. The potential planet g is represented by the darkest blue traces. Tailored from Determine 2 from the paper.

To find out the origins of those thriller dips, the authors used the Spitzer Space Telescope to stare at K2-138 for 11 hours, centered across the predicted transit time of the proposed sixth planet. By becoming the unique K2 and new Spitzer information, a transparent transit occasion, proven in Determine 2, was discovered within the Spitzer observations inside an hour of the anticipated time, confirming the existence of a sixth planet, K2-138g.

Figure showing the transit light curve of K2-138g with two plots next to each other. Both plots share the same y axis "Relative Flux" which varies between 1.002 and 0.996. Both plots have an x axis of "Hours from Mid-Transit", varying between -4 and 4, with the plot centered around 0. On the left, the K2 transit model is shown by a blue line at 1.0 relative flux, dropping down 0.999 during transit. The transit lasts between approximately -2 and 2 hours from mid-transit has a slightly rounded bottom. Two transits are plotted on top of the model, with data points roughly every 20 minutes. The first transit is shown by yellow circles and the second by red triangles, with a relatively large amount of scatter around the blue transit model. The plot on the right shows the Spitzer transit. The transit model is again plotted in blue and has a near identical shape to the K2 model, but this time has a flatter bottom. Grey circles are significantly scattered around the model line showing all the Spitzer flux measurements. To show the transit shape of the data more clearly, the grey points are binned to 20 minutes intervals and shown by red circles with small error bars, closely following the transit model.

Determine 2: The transit mild curves from the K2 and Spitzer observations. Within the left panel, yellow circles and purple triangles present every of the 2 transits seen by K2. In the suitable panel, gray factors present the Spitzer observations. The purple circles present the information binned to twenty minute intervals, displaying the drop in flux brought on by the transiting planet. In each panels, the blue line offers the fitted transit mannequin. Tailored from Determine 5 within the paper.

Orbiting at over twice the gap of planet f, the sub-Neptune K2-138g is one thing of a loner in comparison with its tightly packed siblings. With its 42 day orbit, K2-138g shouldn’t be solely one of many longest interval K2 planets discovered up to now,  makes K2-138 the K2 system with probably the most found planets but. 

Whereas the transit durations of the 2 mild curves in Determine 2 are practically similar, the Spitzer information exhibits K2-138g to have a barely bigger transit depth, and therefore radius. As the 2 transit lengths are constant inside one sigma, the authors word that the restricted variety of information factors within the K2 transits imply that any outliers might skew the outcomes, inflicting the slight discrepancy with Spitzer.

The Extra the Merrier

Whereas planets b, c, d, and e are in close to 3:2 resonance with their respective neighbours, the outer planets f and g should not. Given this reality, together with the sizeable hole in orbital interval between f and g, might there be further planets within the system but to be found? It appears attainable. If the sample of resonances continued past planet f, resonant orbits could be anticipated at durations of round 20 and 30 days, however extra observations are wanted to verify whether or not any such planets exist.

Figure showing the orbital separations of 8 multiplanet systems. Each system is represented by a horizontal line across the figure with a/R* across the x axis from 0 to 175. The systems are in order of stellar size with the largest at the top. Each line has a thickness representative of it's stellar size, and is a different color from a gradient of pink at the top through yellows and oranges until the final system in red. The planets in each system are plotted on the lines to show their orbital separations from their host stars. The systems from top to bottom are Kepler-90, Kepler-11, Kepler-20, K2-138, HD 219134, Kepler-80, TOI-178 and TRAPPIST-1. Kepler-11, Kepler-20, K2 138, HD 219134 and Kepler-80 have planets bunches together at low values of a/R*, with an additional planet at a much larger orbital seperation. The other systems have more uniform spacing between each of their planets.

Determine 3: The orbital spacings of a choice of multiplanet programs, so as of the dimensions of their stellar hosts from largest on the prime. Every system is proven by a colored line with a width equivalent to the dimensions of the host star. Transiting planets are represented by circles scaled to the road width, enlarged 10x for readability. Non-transiting planets are proven in blue. The massive separation between the 2 outer planets of K2-138 is much like that seen within the Kepler-11, Kepler-20, HD 219134, and Kepler-80 programs. Determine 10 from the paper.

K2-138g isn’t distinctive in its socially distanced orbit, nevertheless. Round half of the 9 different exoplanet programs with 6 or extra planets even have a big hole between their outermost planets, as seen in Determine 3. Whereas this obvious pattern may very well be the results of planet formation processes, planets at large orbital radii will be more durable to detect, so observational biases is perhaps at play.

A Benchmark System 

With its tightly packed resonant internal planets and abundance of sub-Neptunes, the authors argue that the K2-138 system is a greater than worthy goal of follow-up observations. The internal planets present a superb alternative to review their potential transit-timing variations (TTVs), discrepancies within the common durations of planets, and observations have already been scheduled. Alongside radial velocity (RV) information, this might allow exact mass measurements and see the potential discovery of further planets. Whereas the planets have atmospheric alerts too small to be studied with the James Webb Space Telescope, they’re prime targets for the European Area Company’s upcoming ARIEL mission. The system’s 5 sub-Neptunes might present a key check mattress for comparative research of the atmospheres of a planet class not seen in our photo voltaic system.

Regardless of the future holds, it actually appears possible that we’ll be listening to extra from K2’s most musical system within the years to come back!

Astrobite edited by Brent Shapiro-Albert

Featured picture credit score: NASA / JPL-Caltech / R. Damage (IPAC)


About Lili Alderson

Lili Alderson is a primary 12 months PhD pupil on the College of Bristol finding out exoplanet atmospheres with space-based telescopes. She spent her undergrad on the College of Southampton with a 12 months in analysis on the Middle for Astrophysics | Harvard-Smithsonian. When not fascinated with exoplanets, Lili enjoys ballet, movie and baking.

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The Von Trapp Family Planets: A Sixth Planet Confirmed for K2-138


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