I am Iron Exoplanet: discover GJ 367b, a dense underground

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Title: GJ 367b: A dense, ultra-short-period subterranean planet transiting a nearby red dwarf star

Authors: Kristine WF Lam, Szilard Csizmadia, et al.

Institutions of the first authors: Technical University of Berlin and German Aerospace Center

Status: Posted in Science, available on arXiv

As special as we like to think Earth is, small planets are all around us – the most common type of star in the solar neighborhood, M dwarfs, should house a average of 2.5 planets less than 4 terrestrial radii. But these planets are not always easy to find. A small radius means that transit signal, the amount of light blocked as the planet passes in front of its host star, is only a tiny fraction of the total stellar flux. Moreover, the radial velocity (RV), a measure of how much the planet’s gravitational pull alters the observed spectrum of its star, is difficult to detect due to the low mass and therefore small gravitational impact the planet has on the star . Detecting such planets around M dwarfs is somewhat easier than around solar stars thanks to the smaller radii and stellar masses of M dwarfs, but it is not always so straightforward. Young M dwarfs can be extremely active, with stellar spots that can be mistaken for transits and flares that complicate starbase flux, complicating transit detections, while the added noise from activity makes selecting an RV signal much more difficult.

However, despite all the challenges, the Transiting Exoplanet Survey Satellite, TESS, at already found several small, rocky exoplanets orbiting M dwarf stars, and should find many more as it continues its extended mission. In fact, the newest of these planets, GJ 367b, is perhaps one of the most intriguing to date!

The GJ 367 system

Figure 1: Adjustment of data for the new exoplanet GJ 367b. The upper part shows the TESS transit light curve and the residuals, with a transit time of 29 minutes. The lower section shows HARPS RV measurements and remnants of the host star, with the black dots showing data clustered at roughly 45-minute intervals. This shows that the maximum velocity change caused by the gravitational pull of GJ 367b was 80 cm-1. Figure 1 in the article.

The M dwarf GJ 367 was observed by TESS in March 2019 and flagged as a potential host planet after its light curve showed a steady decline of 0.03% every 0.32 days. With the signal pointing to a candidate planet at around 0.75 times Earth’s radius, the authors of today’s article worked to verify that the signal was real with ground-tracking observations using of two different telescopes: the Rapid Eye Mount Telescope in La Silla and the Las Cumbres Observatory Global Telescope Network. Using these measurements, they were able to rule out that the signal could have been caused by the fact that the M dwarf was part of a eclipsing binary. They also found that contamination from other nearby stars led to an underestimation of the candidate radius in the TESS lightcurve analysis.

To understand the physical properties of the new planet, the authors combined TESS transits with RV measurements of the HARPS spectrograph at La Silla, and modeled both datasets simultaneously using a Markov chain Monte Carlo code, as shown in Figure 1. The new planet, designated GJ 367b, orbits in 7.7 hours at lightning speed and is close enough to its star for the dayside to reach over 1700K, hot enough to evaporate any atmosphere if the planet was able to hold one. With a radius of 0.718 times that of Earth and having about half the mass of Earth, GJ 367b is one of the smallest exoplanets with well-refined size measurements, and comes in at a super dense density of 8 gcm-3!

East GJ 367b another Super-Mercury?

This iron-like density makes GJ 367b considerably denser than other ultrashort-period exoplanets, as shown in Figure 2, and is unlikely to be a purely rocky or more Earth-like planet. The authors therefore investigated the potential composition using a mixing density network (MDN), a machine learning technique that can be used to predict the composition of planetary interiors.

A plot of the radius on the y axis and the mass on the x axis for the minor planets.  The planets are colored purple - blue - green - yellow according to their temperature from top to bottom, and a key is shown on the left.  The minor planets of the solar system are also traced with black diamonds.  Colored dotted lines cover the plot and show the densities of pure Fe, 80% Fe, 50% Fe, 100% MgSiO3 and 50% H2O

Figure 2: Masses and radii of known small exoplanets. Exoplanets are color-coded according to their diurnal equilibrium temperatures, while planets in the solar system are represented by black diamonds. The dotted density lines represent different possible compositions, with less dense water and rock worlds towards the top, and those with increasingly larger iron cores towards the bottom. Figure 2 in the article.

DND has found that GJ 367b probably has an iron core taking up 86% of its radius, with the majority of the rest being the planet’s mantle. This is similar to the size of Mercury’s iron core measured at 83% by the MESSENGER spacecraft, and also obtained by DND. Although the mass and radius of the planet are well constrained, they still allow a fairly wide range of densities to be plausible, so these results are not certain. However, even at its lowest possible density, the authors note that the iron core’s fractional radius would still be larger than Earth’s, so even though GJ 367b might not be a true “super-mercurythe picture, as always, is a bit more complicated than “Earth-like” would imply.

Astrobite edited by Alice Curtin

Featured image credit: PSP 1992 (Patricia Klein)

About Lili Alderson

Lili Alderson is a second-year PhD student at the University of Bristol studying the atmospheres of exoplanets with space telescopes. She did her undergraduate at the University of Southampton with a year of research at the Center for Astrophysics | Harvard-Smithsonian. When she’s not thinking about exoplanets, Lili loves ballet, cinema and baking.

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