TESS Spies, with Its Little Eye, Something Multi-Planetary

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Title: TESS Spots a Compact System of Super-Earths Around the Naked-Eye Star HR 858
Authors: Andrew Vanderburg, Chelsea X. Huang, Joseph E. Rodriguez, Juliette C. Becker, George R. Ricker, et al.
First Author’s Institution: The University of Texas at Austin
Status: Submitted to ApJL

The Transiting Exoplanet Survey Satellite (TESS) has been operating for over a year now. It is nearly halfway through its survey of the sky, currently observing Sector 11 of 26 (see Figure 1). TESS has already revealed new planets (including an Earth-sized one) and even caught some supernovae as they were getting brighter.

TESS sectors

Figure 1: A map of the sectors observed by TESS in the first year of observations, in celestial coordinates. The thick dark line is the galactic plane; the thin dark line is the ecliptic (the apparent path traced out by the Sun over a year). The different colored squares denote which of TESS’s four cameras is used to observe that part of the sky. [TESS]

The paper discussed in this Astrobite announces another new and exciting TESS detection — not one, not two, but three super-Earths orbiting a bright, nearby star. The host, HR 858, is located in the constellation of Fornax the Furnace and, as a sixth magnitude star, it is just at the edge of what can be seen with the naked eye.

Certainly Not Light on Planets

HR 858

Figure 2: HR 858 as observed by the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) (left) and TESS (right). The purple and red lines demarcate the area used to measure the brightness of the star in Sectors 3 and 4 respectively. The blue line shows the extent of one arcsecond in the observations. [Vanderburg et al. 2019]

HR 858 was observed in Sectors 3 and 4. In Sector 3, it was imaged once every 30 minutes as part of the full-frame images (the entire area one of TESS’s cameras can see) since it was near the edge of the sector. In Sector 4, HR 858 was imaged every 2 minutes, typical for bright nearby stars in TESS’s field of view (see Figure 2).

After correcting for errors (including the accidental activation of an onboard heater), the authors obtained a light curve for HR 858. If HR 858 were hosting any planets and any of those planets passed in front of it while TESS was observing it, the light curve would contain dips that corresponded to the planet transits. Two possible planet signals emerged early in the analysis, with periods of 3.59 and 5.97 days. When the light curves of Sectors 3 and 4 were combined, another candidate popped out with a period of 11.23 days (see Figure 3).

HR 858 light curve

Figure 3: The combined light curve of HR 858 from Sector 3 and 4. The x-axis shows the Barycentric Julian Date minus 2457000 days, and the y-axis shows relative brightness. The gray points show the observations used to construct the light curve. The dips in the purple line are the planet transits. For visual purposes, the 2-minute observations in Sector 4 were binned to match the 30-minute observations in Sector 3. [Vanderburg et al. 2019]

The authors ruled out false positives with archival data and follow-up observations. They found that any nearby stars were too faint to significantly impact the brightness of HR 858. Spectroscopic observations proved that HR 858 was not part of a binary star system, cementing the planet candidates as actual planets. However, the authors did notice a faint stellar companion to HR 858, HR 858 B, that moves at roughly the same speed.

A Bright Future

The planets — HR 858 b, c, and d — all have fairly short orbital periods and so are very close to their host star. Fitting their transits showed that all three were super-Earths, about twice as large as the Earth. Compact systems of rocky planets are not unheard of, but what sets this system apart is that HR 858 b and HR 858 c may be in mean motion resonance (MMR). This means that the orbital periods of the two planets are in an integer ratio with each other (specifically 3:5 for HR 858 b and c). Compact multi-planet systems in MMR are few and far between, and since MMR may play a role in planetary formation, this prospect in HR 858 is worth investigating.

There is also the possibility that the orbital plane of the planets is misaligned relative to HR 858’s own axis of rotation. The authors speculate that HR 858 B may be responsible, having interacted with the disk that formed HR 858’s planets. Long term follow-up observations should be able to verify this, as well as the likelihood of MMR between HR 858 b and c.

HR 858 is the brightest multi-planet host we have detected so far (see Figure 4). This makes it rich ground for several follow-up studies; it can definitely aid us in better understanding the interactions between stars and their planets.

Known systems with at least three transiting planets

Figure 4. Known systems with at least three transiting planets. The x-axis shows planet orbital period and the y-axis shows the apparent Gaia magnitude of the host star. The circles indicate the planets in each system and their relative sizes. The color of the circles in a system indicates the temperature of their host star. HR 858 is the brightest star in this plot. Any planets in MMR are highlighted with purple outlines. [Vanderburg et al. 2019]

About the author, Tarini Konchady:

I’m a second year graduate student at Texas A&M University. Currently I’m looking for Mira variables to better calibrate the distance ladder. I’m also looking for somewhere to hide my excess yarn (I’m told I may have a problem).