What kind of planet is Proxima Centauri b?
Proxima Centauri b is a confirmed planet. It completes an orbit in about 11.18 days at an average distance of roughly 7.25 million kilometres from its star. The smallest mass allowed by the observations is about 1.06 times Earth’s mass. But the planet has not been seen passing in front of its star, so its size, density, atmosphere and surface have not been measured directly. An Earth-like mass and a place in the habitable zone do not by themselves make it an Earth-like world.
The planet was not discovered in a photograph or by seeing it block starlight. Its gravity pulls Proxima Centauri slightly back and forth, producing a tiny difference in the color of the star’s light as the star moves toward and away from Earth. That change repeated about every 11.18 days, and the same period was found in visible-light and near-infrared observations.
- About 11.18 days Time Proxima b takes to orbit its star once
- About 7.25 million km Average distance between the star and planet
- About 1.06 Earth masses Smallest mass allowed by the observations
- About 64% of Earth’s Starlight energy received compared with Earth
How can we determine the mass of a planet we cannot see?
When Proxima b pulls on its star, the star also moves by a tiny amount. From Earth, the star’s speed in one direction changes by up to about 1.23 metres per second. Researchers use subtle changes in starlight to measure how quickly the star is moving toward or away from Earth. The timing and size of this repeating motion reveal the planet’s orbit and possible mass.
This method does not immediately show how much the planet’s orbit is tilted to our line of sight. It measures only the part of the star’s motion directed toward or away from Earth. The resulting mass is therefore the smallest possible value, not necessarily the true mass. If the orbit is steeply tilted from our viewpoint, the actual mass could be greater than 1.06 Earth masses.
Dark spots and bright regions on the star can also change its light and create a signal that resembles a planet. Researchers compared the star’s brightness, magnetic activity and speeds measured in several kinds of light. They checked whether the 11.18-day signal repeated with different instruments, and Proxima b is now classified as a confirmed planet rather than a candidate.
Why are its size and appearance still unknown?
When a planet passes in front of its star, the fraction of starlight it blocks reveals its radius. No such transit has been confirmed for Proxima b. Without a radius, researchers cannot calculate its average density by dividing mass by volume. They also cannot tell whether the planet is mostly rock, rich in water or wrapped in a thick layer of gas.
The radius of about 1.02 Earth radii shown in NASA’s catalogue is an estimate, not a direct measurement. It is based on the sizes of other exoplanets with similar masses. The planet’s size and surface illustration on this page likewise use that estimate and several possible compositions. They are not photographs showing real oceans, clouds, continents or colors.
Proxima b’s orbital period, orbital distance, minimum mass and received energy are known. Its radius, density, atmosphere and surface have not yet been measured.
What does it mean to orbit in the habitable zone?
Proxima b receives about 64% as much energy from its star as Earth receives from the Sun. It lies at a distance where liquid water might exist on the surface if an atmosphere traps the right amount of heat. This region is called the habitable zone, but being there does not mean that water or an atmosphere has actually been found.
Without an atmosphere, the planet’s temperature would depend strongly on how much starlight its surface reflects and how it releases heat on the day and night sides. With an atmosphere, the kinds and amounts of gas, air pressure, clouds and winds would change the temperature again. The catalogue value of about 218 K, or roughly −55°C, is not an observed surface temperature. It is a comparison value calculated by assuming no atmosphere and a specified way for heat to spread.
Because the planet is so close to its star, it is likely to rotate once per orbit and keep the same side facing the star. This is called synchronous rotation, but Proxima b’s rotation has not been measured directly. If it does rotate synchronously, a substantial atmosphere could carry heat from the day side to the night side. Without an atmosphere, the temperature contrast between the two sides could be much larger.
The habitable zone is a range defined by distance from a star and the amount of starlight a planet receives. It is not a sign that liquid water, oxygen, life or a human-habitable environment has been confirmed.
Could an atmosphere survive the star’s powerful flares?
Proxima Centauri frequently produces flares rich in ultraviolet light and X-rays. High-energy radiation can break apart atmospheric molecules and help some particles escape the planet’s gravity into space. Because Proxima b is close to its star, this process could have a major effect over long periods if an atmosphere is present.
A 2017 NASA study did not observe Proxima b’s actual atmosphere. It modelled an imaginary planet with Earth-like atmosphere, gravity and magnetic field placed in Proxima b’s orbit. When the researchers changed the upper atmosphere’s temperature and how readily gas could escape, the time required to lose the equivalent of one Earth atmosphere ranged from about 100 million to 2 billion years. The result changed greatly with the starting assumptions.
We do not know whether Proxima b began with a thicker atmosphere, whether activity inside the planet replenishes gases or whether it has a magnetic field. Nor is there evidence that an atmosphere remains today. Current observations therefore support neither “its atmosphere is gone” nor “it has an Earth-like atmosphere” as a confirmed statement.
- Measure the star’s wobble Use its back-and-forth motion to find the orbital period and the smallest possible mass.
- Calculate the starlight the planet receives Compare it with Earth using the star’s brightness and the orbital distance.
- Look for a transit across the star None has been found, so the planet’s size, density and atmosphere cannot be measured directly this way.
- Calculate several possible conditions Compare how temperature and atmospheric loss change under different assumptions for atmosphere, rotation and magnetic field.
- Continue with other observing methods Separate planetary light from starlight to search for an atmosphere and use the star’s position changes to narrow down the true mass.
What observations could tell us more about Proxima b?
Because Proxima b does not pass in front of its star, the usual method of measuring starlight filtered through a planet’s atmosphere cannot be used. The planet is far dimmer than the star and appears immediately beside it in the sky. Studying an atmosphere would require a large telescope to suppress as much starlight as possible, separate the planet’s light and search it for the signatures of particular gases.
Precise measurements of the star’s motion across the sky could narrow down the tilt of the planet’s orbit and its true mass. Long-term observations of the star’s back-and-forth motion can better separate changes caused by starspots and flares from those caused by the planet. Proxima b matters not because it is known to resemble Earth, but because this nearby small planet shows how different confirmed measurements can be from properties that still depend on models.
Sources
- NASA Exoplanet Archive — Proxima Centauri b
- NASA Science — Proxima Centauri b
- ESO — Planet Found in Habitable Zone Around Nearest Star
- Astronomy & Astrophysics — ESPRESSO Analysis
- Astronomy & Astrophysics — Proxima Centauri with NIRPS
- NASA Science — An Earth-like Atmosphere May Not Survive Proxima b’s Orbit
- NASA Science — Neighboring Star’s Bad Behavior