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Astronomers detect planets around Proxima Centauri by measuring how the planets’ gravity makes the star move. High-resolution spectrographs record tiny, repeating Doppler shifts in Proxima’s spectral lines as the star alternately moves toward and away from Earth. The period and amplitude of that reflex motion reveal an orbit and a minimum planetary mass. Proxima b and Proxima d were discovered this way, using the HARPS and ESPRESSO instruments; neither discovery depended on a confirmed transit. Proxima c remains a less secure, long-period candidate rather than an equal-strength confirmation.
What is actually being detected?
Proxima Centauri is far too close in apparent angular separation to its planets for ordinary telescopes to photograph those worlds directly. The established detections come from the star’s response to its planets, not from seeing reflected planetary light.
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A planet and its star orbit their shared center of mass. Because the star is much heavier, its path is a small wobble while the planet traces the larger orbit. As the star’s motion carries it slightly toward Earth, wavelengths in its spectrum shift toward blue; as it moves away, they shift toward red. This is the Doppler effect. The velocity is only a tiny fraction of the star’s overall motion, so astronomers need stable spectrographs, repeated observations and careful modelling.
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A genuine planetary signal has a consistent period and amplitude over many observations. The period gives the orbital timescale. The amplitude, combined with the star’s mass and the fitted orbit, gives a minimum mass written as m sin i, where i is the unknown orbital inclination. If the orbit is not viewed edge-on, the true mass is larger than the radial-velocity minimum.
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How radial-velocity detection works
1. Build a time series of spectra
Observers point a high-resolution spectrograph at Proxima repeatedly, often over nights, months or years. Each exposure contains many stellar absorption lines. Comparing those lines with a reference spectrum allows the team to measure the star’s line-of-sight velocity at each observing time.
2. Search for a stable period
The measured velocities are tested for periodic patterns. A planet should produce a coherent cycle: the star accelerates toward one side of the system, reaches a velocity extreme, reverses, and then moves the other way. A period that appears only during a flare, or that changes with the star’s activity, is not sufficient evidence.
3. Fit an orbital model
An orbital fit estimates the period, velocity amplitude and—in cases where the data support it—eccentricity and phase. For a radial-velocity-only detection, the fitted mass is a minimum because inclination is normally unknown. The measurement does not by itself provide a physical radius or a bulk density.
4. Test stellar alternatives
Proxima is an active red dwarf. Flares, spots and changing surface regions can distort spectral lines or create apparent velocity shifts. Teams therefore compare the proposed period with photometric brightness changes, flare times, line-shape indicators and other activity diagnostics. Measurements taken during obvious flares may be removed, and the remaining signal must still persist.
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5. Confirm with independent precision
A second instrument or a higher-stability spectrograph can test whether the signal survives different hardware and analysis choices. For the Proxima system, ESPRESSO on the European Southern Observatory’s Very Large Telescope supplied an important higher-precision check on earlier HARPS-era measurements.
Proxima b: the first widely accepted planet
The discovery of Proxima b was announced in 2016 after a campaign with the HARPS spectrograph on ESO’s 3.6-metre telescope at La Silla. The team combined repeated radial-velocity measurements with brightness monitoring. ESO states that “Radial velocity data taken when the star was flaring were excluded from the final analysis.” That filtering mattered because activity could otherwise imitate a planetary period.
The resulting signal repeats every roughly 11 days. NASA’s 2018 catalog entry lists Proxima b at 1.07 Earth masses and an 11.2-day orbital period, with radial velocity as the discovery method. The mass figure is the catalog’s reported value; in a radial-velocity context it should be understood as a minimum unless the orbit’s inclination is independently known.
What the discovery established was a recurring gravitational tug with the period expected from a planet. It did not produce a photograph of Proxima b, a measured surface, or a confirmed transit radius. Those are different observations requiring different signals.
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Proxima d: a weaker, shorter-period signal
Follow-up work with ESPRESSO found a much smaller periodic motion. ESO reported in 2022 that Proxima d has about one quarter of Earth’s mass and completes an orbit in about five days. NASA’s current catalog lists 0.26 Earth masses, a 5.1-day period, radial velocity as the discovery method and a 2025 discovery date for its catalog entry. The announcement year and the later catalog date are not the same thing: a catalog can be updated after the original discovery publication.
Because the signal was weak, the team obtained additional ESPRESSO observations to check whether it came from the star itself. ESO described the reason directly: “As the signal was so weak, the team had to conduct follow-up observations with ESPRESSO to confirm that it was due to a planet, and not simply a result of changes in the star itself.” This is a central part of exoplanet detection: significance is not just a large-looking peak in one data set, but persistence under new observations and activity tests.
Why Proxima c is treated differently
Proxima c is associated with a much longer claimed period than b or d. The NASA Exoplanet Archive overview lists an entry of approximately 1,900 days for a radial-velocity signal. However, an ESO-hosted science paper reports that the available ESPRESSO data did not detect c and that characterizing the signal may require continued spectroscopy or astrometric measurements from Gaia.
For readers, the important distinction is status. Proxima b and d are the stronger radial-velocity detections described above. Proxima c should be called a candidate or debated long-period signal, not presented as equally confirmed. A multi-year period is intrinsically harder to establish because it requires a long observational baseline and can be confused with slow instrumental or stellar trends.
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Transit photometry is a different method
In the transit method, a planet passes directly between Earth and its star and blocks a small fraction of the star’s light. NASA describes the geometry as a planet passing “directly between an observer and the star it orbits.” A sensitive photometer would record a dip that repeats at the orbital period.
A transit and a radial-velocity signal answer different questions. The depth of a transit depends mainly on the planet-to-star area ratio and therefore supplies a radius. The timing gives a period, while the transit geometry constrains the inclination. Combining a radius with a mass can yield an average density. But a transit requires an orbit aligned nearly edge-on to our line of sight, so many planets that produce radial-velocity signals never transit from Earth.
| Feature | Radial velocity | Transit photometry |
|---|---|---|
| Signal measured | Doppler shifts in stellar spectral lines as the star moves toward and away from Earth | Periodic decrease in the star’s measured brightness when a planet crosses the stellar disk |
| Main results | Orbital period and a minimum mass, m sin i | Orbital period, transit depth, radius estimate and a direct inclination constraint |
| Geometric bias | Works for a broad range of orbital inclinations, although the inferred mass depends on inclination | Requires an orbit aligned close to edge-on as seen from Earth |
| Dominant noise concern in this case | Stellar activity such as flares and changing surface features | Intrinsic photometric variability, including changes in the star’s brightness |
| Established route for Proxima b and d | Yes; HARPS and ESPRESSO radial-velocity measurements | No confirmed transit is established in the cited material |
Thus, the answer to “Was Proxima d found by a transit?” is no in the published account summarized here: its discovery method is radial velocity. A transit search could still be scientifically useful, but it is not the evidence that established d.
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- Collect a long, high-resolution spectral record. Observers schedule enough measurements to cover the proposed period repeatedly, while recording the exact observing times and instrument conditions.
- Convert line shifts into velocities. The analysis measures Doppler displacement across many spectral features rather than relying on one line. The resulting velocity series is searched for periodic power.
- Fit the orbit and report the correct mass quantity. Period and amplitude are fitted together. Unless inclination is known, the result is reported as a minimum mass, not an unrestricted true mass.
- Compare against activity indicators. Teams examine photometry, flare timing, line widths, line bisectors and related diagnostics. A velocity cycle that tracks stellar activity is suspect.
- Remove or model contaminated observations. In the Proxima b campaign, measurements taken when the star was flaring were excluded from the final analysis. Such filtering must be documented because it changes the data set being tested.
- Repeat with a separate or more precise instrument. ESPRESSO follow-up was especially important for the low-amplitude Proxima d signal and for testing whether the apparent motion was intrinsic to the star.
- Check for a transit when geometry allows. A detected transit would add a radius and inclination. No confirmed Proxima b or d transit is established by the cited sources, so their radial-velocity masses retain the inclination qualification.
What the measurements can and cannot tell us
Mass is not the same as size
Radial velocity measures gravitational influence. It does not reveal whether a planet is rocky, icy or gaseous, because very different compositions can have similar masses. A radius from a transit, or another independent constraint, is needed to estimate density.
The minimum-mass caveat matters
If a planet’s orbit is inclined rather than edge-on, the star’s line-of-sight wobble is reduced. The reported m sin i value is therefore the lowest mass consistent with the measured signal. An orbit viewed nearly face-on could hide a substantially larger true mass, although other observations may constrain that geometry.
Period does not prove habitability
An 11.2-day or 5.1-day period tells us how quickly the planet orbits Proxima. It does not establish an atmosphere, liquid water, surface conditions or biological activity. Those questions require additional measurements and physical modelling beyond the detection itself.
Why Proxima is a difficult target
Proxima’s activity is both an astrophysical subject and a measurement problem. A flare can alter the spectrum and the star’s brightness on timescales that overlap the signals astronomers are seeking. Spots and evolving active regions can shift the apparent centers of spectral lines even when no planet is responsible. The observing strategy therefore combines spectroscopy with photometry and activity diagnostics instead of treating every periodic velocity as planetary by default.
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1Fix the driver behind crashes, sound loss and screen glitches2Repair Windows errors before they cause bigger problems3Scan for outdated or missing drivers - takes under a minuteInstrument stability is equally important. HARPS and ESPRESSO are designed to measure extremely small wavelength changes, but long campaigns still require calibration, consistent data reduction and checks for instrument-specific trends. A signal becomes persuasive when it remains coherent after those checks and after observations with independent precision.
Can a backyard telescope reproduce the detections?
No. A small amateur telescope can potentially contribute brightness monitoring, but the HARPS and ESPRESSO discoveries depended on high-resolution, highly stabilized spectrographs and professional observing campaigns. The radial-velocity amplitudes are far below what ordinary visual observing can measure. A backyard instrument also cannot simply infer a planet from a single night: the evidence requires a timed series, period analysis and stellar-activity diagnostics.
What future observations could clarify
For b and d, further spectroscopy can refine periods, amplitudes and activity models. A confirmed transit would be especially valuable because it would supply a radius and inclination, allowing a density estimate when combined with mass. For c, the key tests are continued long-baseline spectroscopy and astrometric measurements. Gaia-style astrometry measures a star’s position on the sky rather than only its line-of-sight velocity, offering an independent way to test a long-period companion if the signal is large enough.
Quick Recap
The practical answer in one view
- How do astronomers know a planet is orbiting Proxima Centauri? They detect a repeating Doppler wobble in the star’s spectrum, then test its period and amplitude against flares and other stellar activity.
- How did they find Proxima b? HARPS radial-velocity observations, announced in 2016, identified an approximately 11-day signal; NASA’s 2018 catalog lists 1.07 Earth masses and an 11.2-day period.
- How was Proxima d found? ESPRESSO detected and followed up a weaker approximately five-day radial-velocity signal; NASA’s current catalog lists 0.26 Earth masses and a 5.1-day period.
- Was either discovery a confirmed transit? No confirmed transit for b or d is established in the cited material.
- Is Proxima c confirmed? Its approximately 1,900-day archive entry is a debated or candidate long-period signal; available ESPRESSO data did not detect it, and further spectroscopy or Gaia astrometry is relevant.
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