Intriguing System in New Exoplanet Haul

Today’s announcement of the confirmation of over 100 planets using K2 data reminds me of how much has gone into making K2 a success. You’ll recall that K2 emerged when the Kepler spacecraft lost function in two of its four reaction wheels. Three of these were needed for pointing accuracy, but ingenious pointing techniques and software updates have made K2 into a potent project of its own. The latest announcements demonstrate that certain benefits emerged from the changed mission parameters, especially in the ability of K2 to move away from the original field of view (toward Cygnus and Lyra) and focus on targets in the ecliptic plane.

What we gain from that change is that working in the ecliptic allows more chances for observation from ground-based observatories in both northern and southern hemispheres as they perform the needed exoplanet follow-up. But there are other factors that make K2 potent. With all targets being chosen by the entire scientific community (not limited to the original science team members), we’re drilling down into smaller red dwarf stars. Thus Ian Crossfield (University of Arizona), who is behind the latest tranche of exoplanet discoveries:

“Kepler’s original mission observed a small patch of sky as it was designed to conduct a demographic survey of the different types of planets. This approach effectively meant that relatively few of the brightest, closest red dwarfs were included in Kepler’s survey. The K2 mission allows us to increase the number of small, red stars by a factor of 20 for further study.”

The paper on the new announcement elaborates on the Kepler/K2 distinction:

K2 observes a qualitatively different stellar population than Kepler, namely a much larger fraction of late-type stars [i.e., K and M-class]… Stellar parameters for these late-type systems derived from photometry alone are relatively uncertain, and follow-up spectroscopy is underway to characterize these stars… In addition to the difference in median spectral type, K2 also surveys a much broader range of Galactic environments than was observed in the main Kepler mission. These two factors suggest that, once K2 ’s detection efficiency is improved and quantified, the mission’s data could address new questions about the intrinsic frequency of planets around these different stellar populations.

That factor of 20 increase in small red stars is paying off handsomely. We now have 104 newly confirmed planets, among them a planetary system containing four interesting potentially rocky worlds. Although all four of these planets orbit within the distance of Mercury’s orbit around the Sun, the star itself is an M-dwarf less than half the Sun’s size. The planetary orbital periods go from 5.58 days to 24 days, and according to the paper on this work, “The irradiation levels for several planets are also quite consistent with Earth’s insolation.”

The host of the four planets is the M-dwarf K2-72, all four of whose planets have been validated. The orbital periods here are 5.58, 7.76, 15.19, and 24.16 days, with the authors noting that planets c and d orbit near a first-order 2:1 mean motion resonance, or MMR (in a first-order resonance, the integers in the ratio differ by one), while b and c orbit near a second-order 7:5 MMR. Planetary radii are in the range of 1.2–1.5 R ⊕ for all planets.

All of this is exciting news, though we still have challenges in future observation. The star is faint enough to make Doppler or transit spectroscopy observations, needed to measure planetary mass or perform atmospheric analysis, difficult. It may be that transit timing variations will be helpful in analyzing the masses and bulk densities of these worlds.

Image: A montage showing the Mauna Kea Observatories, Kepler Space Telescope, and night sky with K2 Fields and discovered planetary systems (dots) overlaid. An international team of scientists discovered more than 100 planets based on images from Kepler operating in the K2 Mission. The team confirmed and characterized the planets using a suite of telescopes worldwide, including four on Mauna Kea (the twin telescopes of Keck Observatory, the Gemini-North Telescope, and the Infrared Telescope Facility). The planet image on the right is an artist’s impression of a representative planet. Credit: Karen Teramura/IFA; Miloslav Druckmüller/NASA.

Of the 104 planets, 64 are validated in this paper for the first time, and we still have another 63 remaining planet candidates. The paper tells us that the new discoveries include 37 planets smaller than two Earth radii (2R ⊕ ), and several multi-planet systems. The complete list of these worlds is found in the research paper cited below, which points out that K2 may be able to double or triple the number of small planets detected around nearby stars. 500 − 1000 planets are likely to be discovered in K2’s planned four-year mission.

That’s good news, of course, for future attempts to measure the composition of planetary atmospheres with the James Webb Space Telescope, to be launched in 2018, and it feeds excitement for the upcoming Transiting Exoplanet Survey Satellite (TESS) mission, due for launch next year.. We’re just getting a taste here of what TESS is likely to give us. From the paper:

The size of our validated-planet sample demonstrates yet again the power of high-precision time-series photometry to discover large numbers of new planets, even when obtained from the wobbly platform of K2. Since K2 represents a natural transition from the narrow-field, long-baseline Kepler mission to the nearly all-sky, mostly short-baseline TESS survey, the results of our K2 efforts bode well for the productivity of the upcoming TESS mission. The substantial numbers of intermediate-sized planets orbiting moderately bright stars discovered by our (and other) K2 surveys… will be of considerable interest for future follow-up characterization via radial velocity spectroscopy and JWST transit observations…

The paper is Crossfield et al., “197 Candidates and 104 Validated Planets in K2’s First Five Fields,” to be published in Astrophysical Journal Supplement Series (preprint). A Keck University news release is also available.