An international group of astronomers has discovered two planets around Kapteyn´s, a star close to the Sun with a peculiar history, for it was possibly part of a satellite galaxy that was absorbed by the Milky Way. With an estimated 11.5 billion years of existence, the Kapteyn planetary system is one of the oldest known to man.

“Most planets detected around other stars, many of them gaseous, are hundreds of light year away from Earth,” says Pedro J. Amado, researcher at the Institute of Astrophysics of Andalusia (IAA-CSIC) who took part in the finding-. The challenge, nowadays, is to find rocky planets that lie within the habitable zone, the region around a star where a planet may harbor liquid water.”

Kapteyn´s star, which lies only thirteen light years away from us—making it the twenty-fifth star closest to planet Earth—fills both requirements. Its planetary system is composed of Kapteyn b, a planet about five times more massive than our own which orbits around the star every forty eight days, and Kapteyn c which, with a mass about seven times that of the Earth, has an orbiting period of one hundred and twenty one days. 

Artist conception of the Kapteyn´s star planetary system. Credit: Victor H. Robles, James S. Bullocks, Miguel Rocha (UC-Irvine) and Joel Primack (UC-Santa Cruz).

Kapteyn b is the more promising of the two because it lies in the habitable zone. But the instruments necessary to confirm that it is in fact a world containing water are still in the development phase.

The discovery was made possible by the HARPS spectrograph located in La Silla Observatory (ESO) in Chile. “It is still very difficult to discard false positives in the search for exoplanets, which is why the HARPS data had to be complemented by other spectrographs,” says Pedro J. Amado (IAA). “We will be able to overcome that problem using CARMENES, an instrument we are developing in Calar Alto which, observing the visible as well as the infrared spectrums, will allow us to discard false positives immediately.


Besides their eventual habitability, these planets are interesting because of their unusual history. The Kapteyn´s star is part of a group (the Kapteyn group) located inside the halo of the Milky Way, a spherical structure that envelops the entire galaxy. The group forms a sort of current that spins at a velocity of two hundred and ninety kilometers per second around the center of the Milky Way, but in an inverse direction to the other components of the galaxy.

Kapteyn’s star and its planets likely come from a dwarf galaxy now merged with the Milky way. This video simulation shows the merging and formation of the characteristic tidal streams of stars resulting from such a galactic merging event. Credit: Victor H. Robles, James S. Bullocks, Miguel Rocha (UC-Irvine) and Joel Primack (UC-Santa Cruz).

The dynamics and speed of the Kapteyn group seem to indicate that it was a shred from a minor galaxy torn apart and absorbed by the Milky Way. This hypothesis not only requires the Kapteyn star to be more than ten billion years old—the Sun, in comparison, is only five billion years old—but it also matches other characteristics of the star (low metallicity and activity) which point in the same direction.

Thus, the recently discovered planetary system might have emerged during the initial stages of the formation of galaxies and survived a process of galactic cannibalism, which makes it a prime source of information on the formation of planets.  

“Low mass stars such as Kapteyn´s –with only a third of the mass of the Sun- can be very long-lived, even as old as the universe itself. No other kind of star would allow us to study the evolution of such ancient planetary systems, because by the time they reached that age, other stars would already have turned into red giants and engulfed the planets within their habitable zones, like the Sun will do with the Earth,” Amado (IAA-CSIC) concludes. 

Sad Kapteyn

(Sad Kapteyn is a fictional story kindly produced by Alastair Reynolds to illustrate and support the key elements of this discovery report. The story describes the arrival of a robotic interstellar probe reaching Kapteyn’s planetary system, and a first exploratory survey of its planets in the far future. Alastair Reynolds worked as an astronomer at the European Space Agency, and later he became a full time science-fiction writer. The story is provided on a non-exclusive basis for use within this release only. Copyright remains to Alastair Reynolds 2014).

Hello, Earth. It’s me again.

        I hope you’re receiving my signal loud and clear.

        You’ll be glad to hear that I’ve warmed up after the long centuries of my interstellar cruise phase. Having run a complete health check, I can confirm that all aspects of me are performing nominally. Better than nominally, if truth be told. At the risk of boastfulness, I’m actually in excellent shape. Propulsion, AI core, long-range sensors and instrumentation, navigation and communication assemblies – I couldn’t be in better condition.

        Not bad for a piece of space hardware which has already visited six solar systems, without ever needing to return home. Of course, I can’t take credit for myself. I was just well manufactured – built to endure for thousands of years.

        All the same, thank you for making me.

        Onto business, anyway – and I can’t begin to tell you what I’ve found, out here around Kapteyn’s star! This really is an extraordinary place – a solar system unlike any that I’ve already visited. I wish you could be here with me, seeing things through my eyes.

        I’ve dug into my background files and I understand why you sent me to Kapteyn’s star. Unlike the other systems I’ve visited, this sun and its little family of worlds aren’t part of the normal family of stars orbiting in the disc and bulge of the galaxy. This is a halo star – a member of a dispersed population of stars and star clusters, enclosing the Milky Way in a great thin sphere. It’s entirely possible that these stars were not originally part of our own galaxy, but were torn free of another one after a kind of gravitational collision. And some of these stars are unmeasurably old – more ancient and venerable, perhaps, than any disc stars.

        Kapteyn’s star is so slow-burning, so settled, that even my instruments can’t put an upper limit on its age. It could be nearly as old as the universe.

        And its planets?

        Just as old.

        Make of this what you will – put it down to failing programming if you like – but I feel the age of this place in my bones. All right, my main bus chassis. I don’t have bones; I know that. But believe me, this system feels truly time-haunted. The silence and the stillness are almost unbearable, like an endlessly building pressure. Nothing has happened here for entire turns of the galaxy; nothing will happen. Kapteyn’s star simmers, eeking out its nuclear lifetime. The dead worlds tick around their dead orbits.

        But once, there was something.

        I know, I’ve taken liberties. I should have transmitted my wake-up signal before doing any investigations. But I couldn’t resist myself. You made me to be curious.

        I found signs of civilisation.

        The first planet – Kapteyn b – still lies within the habitable zone of the star, orbiting once every forty eight days. There’s nothing living there now, not even an atmosphere, but once there was a technological culture.

        Yes, the first I’ve found. The reason I was made in the first place.

        How’s that for a discovery?

        The fact is, it wasn’t hard to detect. Cities cover almost the entire surface of that world. Enormous structures – they must have reached into space! Dishes and towers and the remains of what I think must have been space elevators, climbing all the way to synchronous orbit. A moon, its surface covered by the same kinds of architecture. Evidence of colonisation of the second planet, Kapteyn c, in its much colder orbit.

        Wonders beyond comparison, but scoured into a kind of tomblike grey uniformity, after aeons of micrometeorite and cosmic-ray bombardment. Cities as mute as sphinxes.

        And nowhere the slightest sign of life.

        Continent-sized craters mar Kapteyn b, and I wonder if they speak of some truly awesome catastrophe – a cosmic accident, or something worse? Whatever the case, the builders of these cities are long gone. Perhaps they were dead even before Kapteyn’s star was snatched from the clutches of its mother galaxy.

        At the risk of inferring too much from too little data, I can’t help indulging in a little speculation. I too was the product of a technological civilisation, with the capability to transform a planet, to colonise other moons and worlds, to build daunting structures. The people of Kapteyn b were clearly more advanced than you, my own builders – but given time, you too could have transformed a world in this manner.

        Something to think about, isn’t it?

        Well, that’s me signing off for now. I’m going to do some more exploring of this system, and perhaps drop some instrument packages down onto Kapteyn b itself. There’ll be a risk in that, since I’ll need to come in on quite a tight orbit, and who knows what will happen? Still, that’s a hazard I’m prepared to accept. You made me for this, and I’m grateful for all that I’ve been allowed to see and do.

        But look.

        I know it’s a small thing, and I really shouldn’t bother you about it. But it’s been quite a long while since I heard from you. I put rather a lot of effort into these transmissions, and it would be good – just once – to know that there was someone at the other end, listening in.

        Just a word, to let me know that you still care?

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