Vulcan II: The Wrath of Stellar Activity

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Title: The Death of Vulcan: NEID Reveals That the Planet Candidate Orbiting HD 26965 Is Stellar Activity
Authors: Abigail Burrows et al.
First Author’s Institution: Dartmouth College and NASA’s Jet Propulsion Laboratory
Status: Published in AJ

It’s possible you may have heard of HD 26965, otherwise known as 40 Eridani A, the stellar host for the fictional planet Vulcan (homeworld of Spock, of Star Trek fame). Back in 2018, two teams of astronomers announced the likely presence of a super-Earth — Vulcan, perhaps — orbiting HD 26965, based on radial-velocity measurements from a variety of instruments. These astronomers measured the planet’s mass to be equivalent to more than 8 Earths and the orbital period to be roughly 42 days. But before Trekkies could break out the Romulan ale, researchers noted that more work was needed to fully separate the radial-velocity signals from those of the exoplaneteer’s eternal enemy: stellar activity, which can mimic planetary signals.

The Fate of Vulcan

Since then, study after study has highlighted stellar activity as the likely primary source of Vulcan’s radial-velocity signal, raising further uncertainty about the existence of the planet. What might bring us closer to a definite answer would be to combine detailed radial-velocity analysis techniques with what we already know about the effects of solar activity on radial-velocity measurements. This knowledge can be applied to high-resolution spectra of HD 26965 taken by NEID at the Kitt Peak National Observatory. NEID represents the latest and greatest in ground-based spectrometers, ensuring clearer, more frequent data than previous studies had access to.

Important to determining the status of HD 26965 b (the standard name for our Vulcan) is the concept of phase lag. Based on what we know from studying solar activity, the observational parameters used to study stellar activity can become offset in time from the effects of stellar activity on radial-velocity measurements. This would mean that stellar activity might not have been fully corrected for in early studies of this planet. If a decaying starspot or plage were present on HD 26965, correcting for it would lead to a reduction in the radial-velocity signal.

Combining Radial Velocity Analysis Techniques

To account for this phase lag, the authors first compute what are essentially smoothed radial-velocity signals by taking the sum of every radial velocity corresponding to every spectral line, weighted by their corresponding errors. These are corrected for NEID systematic quirks (such as by converting measurements into the stellar frame from the observatory frame NEID usually works in). This produces a “template” radial velocity that the authors compare each observed spectral line to, noting differences between observed radial velocity and “template” radial velocity. This comparison allows them to view how phase lag may affect the amplitude and location of the radial-velocity signal.

The Hits Just Keep On Coming

The observed NEID radial velocities have a similar 42-day period to the planet model proposed by an earlier study, but Figure 1 shows that they are out of phase by 30–40%! Straight off the bat, it’s not looking so good for Vulcan with strike number one. In addition, comparing these radial velocities to activity indicator lines Ca Ⅱ, H, and K reveals that they share a similar period but do not seem to be correlated. Strike number two!

observed radial velocities compared to the model including a planet in the system

Figure 1: The NEID-observed radial velocities of HD 26965 (marked in purple) compared to the planet model proposed by an earlier study (Ma et al. 2018, marked in yellow). The NEID radial velocities are out of phase with the planet model by 30–40%. [Burrows et al. 2024]

Going back to phase lag, the authors calculate phase offsets for all activity indicators using a Gaussian process, finding a consistent phase lag between 4.65 and 6.67 days, more than 10% of the star’s rotation period. Shifting data with this phase lag leads to significant reduction of the radial-velocity signal strength when modeled with a 42-day period, and overall over a variety of periods, seen in the periodograms in Figure 2. Strike number three!

radial-velocity measurements and periodograms per stellar activity

Figure 2: Left: A variety of corrected radial-velocity measurements per activity metric, marked with purple circles, accompanied by original radial velocities, marked with gray squares. Right: Periodograms for corrected radial velocities, marked in solid purple, and for original radial velocities, marked in gray dashes. The red diamonds denote periods with the highest remaining power. At the supposed 42-day period, the probability of that being the actual period (called the power) drops significantly for most activity indicators. [Burrows et al. 2024]

Stop, Stop, He’s Already Dead!

These are just a small sampling of demonstrations in the article — ultimately, they all seem to point towards stellar activity probably being the source of the radial-velocity signal from HD 26965, unfortunately for Vulcan.

Fortunately for us, this article aims to act as a Swiss army knife of sorts. It accomplished several things — first, the authors showed that phase lags may be important in determining the relationship between radial velocity and whatever may have affected it. Second, a bundling of analyses makes for a stronger case compared to a single analysis technique on its own, giving more solid evidence for the stellar activity hypothesis.

Last but not least, the methodologies used in this article can be used on radial velocities from other stars, including our own Sun! In addition to testing this multi-pronged analysis on other types of stars, the authors hope to use it on our Sun during especially turbulent activity periods. By refining our understanding of how our Sun and other stars fluctuate through time, we better our exoplanet detection techniques and our chances of finding habitable worlds.

Original astrobite edited by Dee Dunne.

About the author, Diana Solano-Oropeza:

I’m a first-year astronomy PhD student at Cornell University, where I study exoplanets, stars, and habitability using Gaia data. I earned my BS in physics at Drexel University before entering the Bridge to the PhD in STEM program at Columbia University. There, I researched TESS-detected exoplanets for two years. My hobbies include practicing Muay Thai, writing fiction, and playing video games. You can check out my website at