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: Department of Astronomy, The University of Texas at Austin
Status: Submitted to AAS Journals, open access on arXiv
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.
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 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’ 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’ 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 was hosting any planets and if any of those planets passed in front of HR 858 while TESS was observing it, the light curve would contain dips that corresponded to the planet transits. Two possible planets 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).
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 is 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 mean motion resonance 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.