9.10.2023

Of the thousands of stars visible to the eye, only a few hundred are known to have planets. But that number may be far higher in reality

Our galaxy teems with planets of all sorts, including ones orbiting some of the brightest stars in Earth’s sky. Credit: NASA/JPL-Caltech

I remember quite clearly when we only knew of nine planets in the entire universe.

That was the case on the first day of 1992, but scarcely a week later, everything changed. On January 9 of that year astronomers announced the discovery of the very first exoplanets—worlds orbiting stars other than our own. These new planets are so weird that it was difficult to grasp how profoundly they changed our cosmic context: they orbit a pulsar, a rapidly spinning, ultradense, city-sized stellar remnant left behind after a massive star exploded as a supernova. Although that’s extremely interesting, it’s not entirely satisfying. A pulsar is the least sunlike kind of star out there, and we, as irredeemably self-centered human beings, prefer to find places more like home—planets around stars more like our own.

Then, in 1995, astronomers announced they’d found one: 51 Pegasi b, a Jupiter-class planet circling a star that very much resembled our sun in size, mass and age.

And at just a hair more than 50 light-years away, 51 Pegasi is a naked-eye star.

It’s not easy to spot—at a magnitude of 5.5, you need a dark site and a moonless night to see it—but that does raise an interesting question: How many naked-eye stars host planets? It’s surprisingly challenging to definitively answer this question, as we’ll see, so let’s slightly rephrase: How many naked-eye stars host planets that we know about?

Finding the answer appears to be a snap because you can look it up in databases such as NASA’s Exoplanet Archive, which offers a treasure trove of information on alien worlds. Searching NASA’s database for planets orbiting naked-eye stars (that is, ones with a magnitude of less than 6.0) yields a pretty specific answer: 183.

Because nothing in science is ever that cut-and-dried, however, it’s not quite that simple. For example, three of these planets remain officially unconfirmed, bringing the number down to 180. And at least 20 of them are each about 13 times heavier than Jupiter, the largest and most massive planet in our solar system. That means they skirt the border of being objects called brown dwarfs, which are beefier than proper planets but don’t have quite enough oomph to ignite nuclear fusion in their cores to become true stars. With that in mind, the actual number of naked-eye stars hosting known exoplanets is probably around 150 to 160.

Many of these are easy to see and are among the brightest stars in the sky.

The most brilliant in this sample is Aldebaran, a star 66 light-years from Earth that marks the eye of the bull Taurus. It’s a red giant, a star that was once much like the sun but is starting to bloat and die, having run out of hydrogen fuel in its core. Its planet, Aldebaran b, is a gas giant orbiting the star at about the same distance that Mars orbits the sun. Given that the star is more than 400 times more luminous than the sun, Aldebaran b is getting cooked.

Pollux, one of the twin stars at the head of Gemini (the other being the almost equally bright Castor), is another quite bright exoplanetary host. At 34 light-years from us, it’s closer to Earth than Aldebaran is, and it’s also a red giant. Pollux’s planet, Pollux b, is another gas giant that’s also getting blasted by intense stellar light.

The closest naked-eye star known to have planets is Tau Ceti, a mere 11.9 light-years from our world. It hosts at least four planets, which range in mass from about two to four times that of Earth. These planets are so-called super-Earths—they are more massive and larger than Earth but are perhaps still-rocky planets that are similar to ours. One of them, Tau Ceti f, even orbits at the right distance from the star to potentially be habitable—under a sufficiently broad definition of “habitable,” that is.

The most interesting thing about this list, however, is the stars that aren’t on it. Aldebaran, the brightest, is only about the 13th brightest star in the night sky. Why haven’t we found planets around the dozen brighter ones?

There are many reasons, actually. One is that they may simply not have planets. Many of these stars are difficult to observe for planets. Some of them are intrinsically variable, for example, meaning they fluctuate in brightness, which can confound exoplanet surveys. Capella, the fifth brightest star (not including the sun) is at least a quadruple star system, with four stars orbiting one another. Two of them are old, evolved stars, like Aldebaran and Pollux, and orbit so closely together that it’s unlikely there’s a planet around either one of them. Perhaps a planet orbits both farther out, but this has not yet been determined.

Sirius, the brightest star in the night sky, has no confirmed planets. That may be because it’s a binary star, and the two stars move around each other on a mildly elliptical orbit, which could destabilize planetary orbits. One of the stars expired long ago, blowing off its outer layers and becoming a dense and tiny white dwarf. Such an event may not totally disrupt a planetary system but is not exactly great for them, either. Although a search for planets in the Sirius system has ruled out any that would be much more massive than Jupiter, less massive ones may yet await discovery.

One oddity on this list of naked-eye exoplanet no-shows involves the famous star Alpha Centauri. It’s actually a triple star system, with two sunlike stars that orbit each other and a third star, called Proxima Centauri, that is much farther out. The aptronymically named Proxima is the closest star to our sun. Despite its proximity, it’s a dim bulb that is so faint that it demands a decent telescope to be seen at all. Neither of the two brighter stars has been confirmed to have planets after intensive searches. Yet Proxima hosts at least two planets, and a third one is strongly suspected. So, in a sense, one of the brightest star systems in the sky, and the closest one to us, hosts planets, but the specific star they orbit is not visible to unaided vision. Although I’m not sure that counts for our list, it’s still a cool situation.

Given that there are about 9,000 stars visible to the naked eye, the fraction of them known to host planets is surprisingly small. A lot of this may be because of how we find planets, however. So far most have been found via the transit method, when we happen to see the planet’s orbit edge on so that, once per revolution, it passes directly in front of the star and creates a mini eclipse. The amount of light we see from the star dips a small amount, revealing the planet’s presence.

But as this chancy viewing geometry suggests, most planets will never transit as seen from Earth, which renders them effectively invisible to this workhorse detection technique. So from a statistical standpoint, our searches to date have missed a large fraction of existing planets. Present estimates are such that the actual number of planets out there could be at least 10 times higher. The meaningful takeaway of all this is that most stars in the galaxy probably host planets. We just haven’t seen them yet—at least not with the transit method or any of the various other methods that have added smaller numbers of worlds to our galactic tally.

Think on that the next time you’re outside at night under a clear sky. It may well be that nearly every star you see has planets. The odds are that trillions of them exist in the Milky Way alone. From being able to count all the known planets on two hands to cataloging thousands to positing the existence of trillions, we’ve made incredible progress in our census of worlds in the past 30 years.

If there is a single astronomical fact that makes my heart pound a little harder and fills it with wonder and joy, that may very well be it.

Quelle: SCIENTIFIC AMERICAN