One of the lowest mass planets ever discovered has been detected orbiting Proxima Centauri, the Sun’s nearest neighbor. At a time when detecting small rocky planets remains a challenge for astronomers, Proxima d has just a quarter of the Earth’s mass. Its discovery confirms this is indeed a star system worth visiting, if plans for interstellar travel can just be brought to fruition, even though the promising signal detected from that direction was an error.
Proxima Centauri is such a dim star it is invisible to the naked eye, despite its relative closeness at 4.2 light-years away. Combined with a location too far south for most Northern Hemisphere telescopes this led to it being somewhat neglected as a system to study. That has all changed in recent years, and the Pale Red Dot project hit paydirt in 2016 with the announcement of a planet that was not only similar to Earth mass, but also lies within Proxima’s habitable zone, at least theoretically.
The announcement of Proxima b was followed four years later with the suspected discovery of the vastly more distant Proxima c. Now a paper in Astronomy and Astrophysics announces Proxima d, an even smaller planet, orbiting exceptionally close to the star itself.
At just 4 million kilometers (2.5 million miles), the Proxima-Proxima d distance is only a tenth of that between the Sun and Mercury, and orbiting takes just five days. Proxima’s low luminosity means this location is not the fiery hellscape it would be in our own Solar System, but temperatures at Proxima d’s surface are still expected average 90ºC (190ºF).
Nevertheless, the finding has stirred excitement for the potential it indicates for further discoveries around this and other stars. “The discovery shows that our closest stellar neighbor seems to be packed with interesting new worlds, within reach of further study and future exploration,” said first author Dr João Faria of Portugal’s Instituto de Astrofísica e Ciências do Espaço in a statement.
Proxima is a notoriously flary star. This has led to debate about whether it has a habitable zone at all, or whether its outbursts would strip away any atmosphere a planet like Proxima b might start with. It also delayed confirmation of Proxima d. Observations taken with the European Southern Observatory’s Very Large Telescope confirming Proxima b provided a hint of an additional planet, but the eruptive nature of the star itself obscured the data.
Faria and co-authors have collected the additional observations that allow them to be confident Proxima d exists, despite its low mass creating a wobble of just 39 cm/s (1.4 kmph or 0.9 mph). This is the lightest planet ever detected using radial velocity (also known as the Doppler wobble) method.
A baking-hot world not much more than twice the mass of Mars probably lacking an atmosphere isn’t very inviting. However, the ESO's Dr Pedro Figueira argued its significance lies in the fact we could find it at all. “It shows that the radial velocity technique has the potential to unveil a population of light planets, like our own,” Figueira said.
Proxima’s closeness means telescopes such as the JWST and the future Extremely Large Telescope may be able to reveal information about its planets in a way that will be impossible for more distant stars for a long time. Moreover, if the plan of laser-launching tiny cameras to a fifth of light speed proves practical, we may have a mission go there in the lifetime of people alive today, something unlikely for more interesting, but more distant, planets.