Astronomers have discovered a planet resembling a giant, frozen Earth circling one of the closest stars to the Sun.
The exoplanet is at least 3.2 times as massive as our own world and was confirmed using observations collected by telescopes around the world over 20 years.
The discovery team was led by Ignasi Ribas of Spain’s Institute of Space Studies of Catalonia, and Guillem Anglada Escudé, of Queen Mary University, London.
The planet is the second closest rocky one to be found beyond our own Solar System, and orbits its own sun, a red dwarf star, in about 233 days.
The red dwarf, known as Barnard’s Star, lies only six light-years away and is about nine billion years old, twice the age of the Sun. It is the closest lone star to the Sun and also the fastest moving star in the sky.
Dr Escudé has previously found a rocky planet orbiting the closest star to the Sun, Proxima Centauri, which is part of the Alpha Centauri multiple-star system. The finds are part of the Red Dots search for exoplanets.
The new planet received only a fiftieth as much energy from its home star as Earth gets from the Sun. That means that, although it lies more than twice as close to its star as we do, it is likely to be a frozen and gloomy world, with surface temperatures a chilly -170C.
That is too cold for life as we know it to exist. However, Dr Escudé said that other sources of heat or a dense greenhouse-like atmosphere might make conditions more hospitable.
Astronomers used high-precision instruments on powerful telescopes to search for rocky planets orbiting red dwarf stars. They included spectroscopes on telescopes operated by the European Southern Observatory (ESO) in Chile.
The new planet, known simply as Barnard’s Star b, or GJ 699b, revealed its existence by a tiny wobble in the star’s light, caused by the planet’s gravitational pull. This technique is known as the radial velocity (RV) technique.
Evidence was painstakingly collected over many years. Dr Ribas said: “After a very careful analysis, we are over 99 percent confident that the planet is there.”
Dr Escudé said he had had a pretty good idea that the planet existed two or three years ago. He added: “The combination of instruments was key to allowing us to cross-check our result.”
Skymania’s reporter asked Dr Escudé what it might be like on the surface if the new planet. He replied: “We have no clue, but if you want to speculate, I would put my money on a super-Titan object, but with an even more massive atmosphere.
“Conditions at the ‘rocky’ surface are hard to extrapolate because no one has done much modeling of these kinds of objects. There would be chances to have some sort of ‘hosptitable’ conditions at the surface depending on sources of heat and strength of greenhouse (which heavily depends on what the atmosphere would be made of).”
He added that there are hints of another giant gas planet, more like Jupiter, taking more than ten years to orbit Barnard’s Star, but that still has to be confirmed.
We asked how it felt to discover something so interesting so close to Earth, after years of searching.
He said: “It obviously feels good. But for us the ‘discovery’ has come in many little steps, some of them a bit painful. We had a pretty good idea of the likely existence of the planet since 2-3 years ago. Last year has been mostly about looking at all sorts of possible systematic errors in the data. We identified and corrected quite a number of them, also on historical data.
“At times things looked way grimmer, but the same period and mass was coming all the time. With the addition of more observations we though it was time to get it out and let the community react/comment on it.”
Dr Escudé added: “The detection of the signal is rather strong, the variability is very real, but there are some models for correlated noise that reduce the statistical significance of the detection (although they do not remove it completely, and the same period keeps coming back).
“We will keep monitoring the star, but reporting the detection is important now given that space and ground-based telescopes able to image these planets are being designed and build. In a sense it is a call of attention to the community to get their hands dirty.”
We asked whether there might be more planets in the Barnard’s Star solar system. Dr Escudé told us: “The RV data contains evidence for a longer period signal that would correspond to a gas giant planet in the system (period greater thean 10 years), which also need to be reported so people put resources into figuring it out (or detecting it in alternative ways).”
But he added: “As opposed to other red dwarfs, we do not see evidence for a compact planetary system with warm planets on Barnard’s star. It could be that the planets are too small or their orbits too inclined to produce a noticeable RV signature, but for now the star stays in contrast to the now well established ubiquity of compact planetary systems (lots of Kepler ones, and systems like TRAPPIST-1, Luyten’s star, etc.).”
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