Kapteyn’s star — a nearby star that likely formed outside this galaxy — hosts two planets more than twice as old as Earth.
Astronomers have discovered two new exoplanets orbiting Kapteyn’s star, a nearby red dwarf with a complicated history. The findings have been accepted for publication in the Monthly Notices of the Royal Astronomical Society.
Kapteyn b and c are both suspected to be super-Earths, at least 4.5 and 7 times as massive as the Earth, respectively. Kapteyn b circles its host star in 48 days, closer than Mercury’s orbit around the Sun. But because the star is a cool red dwarf, the planet is likely awash in just enough heat to keep water liquid on its surface.
Though most stars in the celestial sphere stay fixed over the centuries, Kapteyn’s star moves across the sky at a fast clip: 8 arcseconds per year, or the diameter of a full Moon every 225 years. (Only Barnard’s Star moves more quickly across the sky.)
Astronomers explain the speed of Kapteyn’s star with a complex origin story. The star began life in a dwarf galaxy that the Milky Way engulfed and shredded 11.5 billion years ago. The interaction would have flung the dwarf’s stars into different orbits. Now Kapteyn’s star is only 12.8 light-years away, while the likely remnant of that long-dead dwarf galaxy, a giant globular cluster called Omega Centauri, lies 16,000 light-years from Earth.
“This discovery actually implies that the Kapteyn's star's planets were formed in another galaxy, literally far, far away,” says coauthor Mikko Tuomi (Carnegie Institution of Washington and University of Chile). “But today, it is one of the closest stars to the Sun. How lucky are we to be able to study such a peculiar star.”
Not only did this planetary system form outside our galaxy, but it’s also the most ancient planetary system known, about 2.5 times older than the solar system. The planets would have formed when the universe itself was only 2 billion years old.
“If intelligent life evolved there, it would be far ahead of us in technology,” Tuomi speculates.
But a word of warning comes from another team led by Ofer Cohen (Harvard-Smithsonian Center for Astrophysics). Kapteyn’s star belongs to a stellar class known as M dwarfs, which appear to be the most numerous type of star by far in the Milky Way galaxy, easily numbering in the billions.
Though red dwarfs likely host plenty of planets, they might be subject to harsh space weather. Cohen’s team modeled the radiation affecting three known planets circling middle-aged red dwarfs and found that even an Earth-like magnetic field could not protect a habitable world from continuous bombardment.
Harsh solar weather would strip the atmosphere of a rocky planet orbiting in a red dwarf’s habitable zone, leaving the planet more Mars-like than Earth-like, Cohen’s team announced on June 2nd at the summer meeting of the American Astronomical Society in Boston.
Spotting Two Needles in the Haystack
The astronomers discovered the planets from La Silla Observatory and Las Campanas Observatory, both in Chile, and the W. M. Keck Observatory in Hawaii. “The fact that all three instruments in different parts of the world agree with our analysis gives us confidence that this is a clear detection,” says coauthor Pamela Arriagada (Carnegie Institute of Washington).
To make the discoveries, the team measured the star’s tiny periodic wobbles due to the planets‘ gravitational tugs. The effect is minute, but advanced instruments allow the team to measure Doppler shifts in starlight as small as 1 part in 300 million, which corresponds to the star moving at just 1 meter per second, roughly walking speed.
Teasing out a single planet’s signal is relatively straightforward. But “when two or more signals overlap, things get tricky, which explains why these planets were not reported long ago,” says lead author Guillem Anglada-Escudé (Queen Mary University of London).
But there were tantalizing hints. The star had been observed by previous teams and had seemed slightly variable, but no periodic signals could be pulled out.
So Anglada-Escudé and colleagues took a second look. “We collected about 60 more spectra (over 22 nights) and then the signals of the two planets showed up very clearly,” says Anglada-Escudé.
Because Kapteyn’s star is so close, it will be an easy target for follow-up measurements. Anglada-Escudé’s team is hoping to soon attempt imaging the planets directly.
The upcoming Transiting Exoplanet Survey Satellite (TESS), slated to launch in 2017, could also take a look. If Kapteyn b transits in front of the star, it will offer us the first observational opportunity to probe the atmosphere of a potentially habitable world. But the chances of a transit are about 1%, says Anglada-Escudé
At 8.9 magnitudes, Kapteyn’s star can be spotted with binoculars or a small telescope from a dark sky. Located in the constellation of Pictor, which can easily be found with one of our Southern Hemisphere star wheels, this red dwarf is best seen from the southern hemisphere.
G. Anglada-Escudé et al. “Two planets around Kapteyn’s Star: a Cold and a Temperate Super-Earth Orbiting the Nearest Halo Red-Dwarf,” to be published in Monthly Notices of the Royal Astronomical Society.