Why We Started With 55 Cancri A

Every measurement project needs a first target. The first target shapes everything — the pipeline, the error budget, the science questions you learn to ask. We chose 55 Cancri A, and we didn't choose it lightly.

The Star

55 Cancri A — designated HD 75732, catalogued as HIP 43587, named Copernicus by the IAU — is a K0 main-sequence dwarf sitting 40.9 light years away in the constellation Cancer. It's metal-rich: [Fe/H] = +0.32, meaning its photosphere contains roughly twice the iron abundance of the Sun. At T_eff = 5196 K it's slightly cooler than the Sun, surface gravity log g = 4.41 places it squarely on the main sequence, and at an estimated age of ~10 Gyr it's one of the older systems we'll study.

None of that, by itself, makes it unusual. What makes it extraordinary is what orbits it.

The Planets

55 Cancri A hosts five confirmed planets — one of the richest multi-planet systems known around a nearby star. The innermost, 55 Cnc e (Janssen), is a super-Earth with a mass of ~8 M⊕ and an orbital period of just 17.7 hours. It orbits so close to its star that its surface is almost certainly molten. JWST detected CO₂ and possible volcanic outgassing in its atmosphere in 2024, making it one of the best-characterized rocky exoplanets in existence.

The outermost planet, 55 Cnc d, has an orbital period of over 14 years — longer than Jupiter's. This system spans four orders of magnitude in orbital distance, from a lava world that completes a year in less than a day to a gas giant taking more than a decade to orbit once.

The Contested C/O Ratio

Here's where it gets scientifically interesting. The carbon-to-oxygen ratio of a stellar host predicts the mineralogy of its planets:

Teske et al. (2013) measured C/O = 0.78 ± 0.08 for 55 Cancri A — just below the carbon-rich threshold, but within error of it. Other studies have found lower values. The disagreement comes down to how you treat the oxygen measurement: the primary oxygen indicator at 6300 Å is blended with a nickel line that must be carefully subtracted, and different groups handle this differently.

The result matters enormously. If Janssen has a carbon-rich bulk composition, its interior could contain layers of diamond under crushing pressure. If it's oxygen-rich, we're looking at a silicate world not unlike a scaled-up version of Earth's mantle. The two scenarios predict completely different internal structures, heat flows, and long-term habitability prospects for any outer planets in the system.

We are going to settle this. With 88 HARPS spectra co-added to S/N ~ 940, our oxygen measurement will have the precision to distinguish between the published values. And we'll publish the full uncertainty budget — every assumption, every correction, every line — so anyone can check our work.

Why HARPS Makes This Possible

55 Cancri A has been observed extensively with HARPS over the years of planet-hunting campaigns. The ESO archive contains 88 public S1D spectra. Co-added, they achieve S/N ~ 940 per pixel at 6000 Å — high enough to detect phosphorus (P I 6034 Å, EW ~ 12 mÅ) at 60σ significance. That's a detection that would be completely invisible in a single HARPS exposure.

This is the first time the full CHNOPS suite — carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur — will be measured simultaneously for this star at this precision. For a system where one planet may have a CO₂ atmosphere and another may be made of diamond, that matters.

The 2010 Connection

There's a personal dimension here too. The Exoplanet Codex started as a senior astrophysics thesis at the University of Montana in 2010. That thesis analyzed three systems, including 55 Cancri A, using ELODIE spectra at R ~ 42,000 — about 3× lower resolution than HARPS. The results were published, the thesis was filed, and the questions didn't go away.

Fifteen years later, with better instruments, better atomic data, better solar abundances, and a proper uncertainty framework, we're going back. The 2010 measurement used Lodders (2003) solar abundances; we're now on Asplund et al. (2021), which revised the solar iron abundance by −0.04 dex. Every result from 2010 will be recomputed and compared. Some will change. Some won't. All of it will be documented.

What Comes Next

The pipeline is in active development. Step 1 — spectrum acquisition and co-adding — is underway. The next Mission Log entry will cover the solar calibration: before we touch 55 Cancri A, we'll run the pipeline on asteroid Vesta (a HARPS-observed reflector of sunlight) and verify that we recover A(Fe)☉ = 7.46 ± 0.05 and C/O☉ = 0.55 ± 0.05. If we don't, the pipeline has a systematic error that has to be found and fixed before any science results mean anything.

We're building this in public. All code is at github.com/damienabraxas/exoplanetcodex. Sign up for updates below — the first Codex entry for Copernicus will be the result.