Why Some Stars Never Form Planets

The planetary paradigm has shifted so quickly and so radically in the last quarter century that is easy to forget that only a few decades ago, one would be hard-pressed to find any professional astronomer who would stake their careers on the idea that most stars harbor planets. But although the overwhelming majority of stars may harbor some form of planet, not all stars are capable of forming planets.

During the first two decades of looking for planets that circle stars outside our solar system, detections came in dribs and drabs. But NASA’s Kepler telescope changed the paradigm completely when based on Kepler data, astronomers made the statistical case that at least half of all stars in our own Milky Way galaxy must harbor planets.

But what about stars that will forever remain barren of planets?   The cosmos appears to have more than a few of those. 

Thus, here’s a quick survey of which stellar spectral types are best suited to form planets over the range of O, B, A, F, G, K and M stars.

O- and B-type stars —- the most massive, hottest and short lived stars in the cosmos.

We know essentially nothing about the planet population orbiting these stars because their large size, brightness, and spectral properties make the detection of exoplanets using current techniques nearly impossible, Benjamin Fulton, an astronomer at the NASA Exoplanet Science Institute at Caltech, told me.  They only live about 100 million years, which is thought to be about the same timescale needed for terrestrial planets to form, he says. 

Such O and B stars are so powerful that they likely quickly blow away the gas in their circumstellar disk and prevent the disk from cooling sufficiently to allow planets to form, says Fulton.

Even if massive stars did manage to form planets in their wide habitable zones, there would be no time for the planet for develop life before the star became a supergiant and enveloped the planet, Edward Guinan, an astronomer at Villanova University in Pennsylvania, told me.

A- and F-type stars —- are slightly cooler than the O- and B-type stars and more numerous.  A-type stars appear white in color while F stars are yellow-white. 

These stars share many of the same difficulties as O and B stars, but are slightly more friendly to their formation, says Fulton. We have discovered a handful of planets orbiting these stars but know little about the population in general, he says. In theory, many A stars are tame enough and live long enough to host planets, Fulton notes. The real problem is that those planets are very difficult to detect so our current knowledge of the planet population orbiting A stars is very limited, he says.

G- and K-type stars —- We orbit a G-type yellow dwarf star and K-type orange dwarfs have been proposed as perhaps the best suited for the evolution of intelligent life. That’s because they although they share many of our star’s characteristics, they have lifetimes almost double that of our own star.

These stars are very similar to the Sun and are amenable to the detection of a variety of exoplanets, says Fulton.  Since these stars are like our Sun we expect them to be relatively friendly environments for the formation of planets, he says.

M-type stars —- These Red Dwarf stars are the most numerous and longest-lived stars of any in the Universe, as common as fireflies on a warm summer night.

These stars are low-mass, low-luminosity, and small in size which make them best suited to the detection of exoplanets, says Fulton. It appears that small planets are quite common around M dwarfs, but larger giant planets are nearly non-existent, he says.

As for planets around double (binary) stars? 

Very close stellar binaries with solar system-scale separations do indeed appear to disrupt planet formation, says Fulton. The few we do know about orbit the pair of stars leading to Tatooine-like planets with two suns, he says. 

And would stars in globular clusters (compact spheres of gravitationally-bound old stars in a galaxy’s halo) lose their planets due to the fact they were formed in such close proximity to each other? 

It’s more likely that planets have a hard time forming in globular clusters because the crowded environment at early times disrupts the planet-forming discs, says Fulton. There are a few known rogue planets roaming the galaxy and it is unclear how or where these formed, he says.

A hundred years ago, no one dreamed that planets would orbit almost every star. The reality is that planets around sunlike stars appear to be as common as dirt. Does that mean that life in the universe should be ubiquitous?

Although planets appear to be very common, the vast majority of these are not friendly to life as we know it, says Fulton. 

And the Kepler Mission indicates that many planetary systems are gravitationally unstable, says Guinan. This indicates that planets are frequently ejected while some other planets end up getting thrown into the host star, he says.

They are either too hot, too cold, too big, too small, or have no solid surface, says Fulton.

“The number of planets that could potentially host life is much, much smaller than the number of planets that exist,” said Fulton.

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