An illustration of exoplanets. Depositphotos.

It Turns Out That All Stars Don’t Have Planets Orbiting Them

It was previously believed that ALL stars in the galaxy had at least ONE planet orbiting them.

How many stars are there in our galaxies and the universe, and how many planets are these galaxies home to? Do all stars across the universe have planets around them? Or are some stars alone?

It was previously believed that ALL stars in the galaxy had at least ONE planet orbiting them.

But while there are stars that are not orbited by any planets, there are also planets that travel across space without a sun.

In fact, experts estimate that for every planet that orbits a star like our own, there are likely thousands of ‘orphan exoplanets’ wandering across the galaxy alone.

Planets, stars, and Kepler

For years, Kepler observed more than 100,000 stars to look for transits of planets and arrived at a startling conclusion: most stars have at least one planet. Data from the first 5000+ exoplanets discovered show something shocking: 99.9% of them are orbiting metal-rich stars; metal-poor stars are overwhelmingly planetless.

As a result, the vast majority of stars in the Universe don’t have any planets, and rocky, potentially habitable planets are the result of billions of years of cosmic evolution, according to astrophysicist Sethan Siegel.

We previously thought that the Milky Way galaxy likely has ten planets per star on average, though this estimate is based on incomplete data. True averages may range from 3 to 30, but then again, we do not have enough data to come to this conclusion.

A guessing game

Exoplanet studies reached a milestone just a few months ago: over 5000 confirmed exoplanets have been discovered. But as revealed by Siegel, it turns out that we have vastly overestimated the number of stars that have planets after all when we take a closer look at the known exoplanets. as of writing, there are 5,069 confirmed exoplanets and 8,833 candidates.

The reason has to do with the big bang, all the elements that existed at that point, and hundreds of thousands, perhaps millions of years later.

The Big Bang is believed to have “birthed” the Universe 13.8 billion years ago, also creating the earliest atomic nuclei within 3-4 minutes of its creation.

During the next few hundred thousand years, it remained too hot for neutral atoms to form and too cold for further nuclear fusion. The decay of radioactive isotopes, including tritium and beryllium, would still occur and bring an end to all unstable isotopes.

A heavy metal play

After neutral atoms formed, we had a Universe composed of, in terms of mass: 75% hydrogen, 25% helium-4, ~0.01% deuterium (a stable, heavy isotope of hydrogen), ~0.01% helium-3 (a stable, light isotope of helium), and ~0.0000001% lithium-7.

According to Siegel, the only element in the Universe that falls into the “rock and metal category” is lithium. Because only one-part-in-a-billion of the Universe is composed of substances other than hydrogen or helium, we can be confident that the very first stars made of this pristine material could not have formed planets by accreting into them.

Thus rocky planets were simply not possible at the beginning of the Universe and for quite some time afterward.

Simply realizing that is revolutionary in and of itself, and it honestly does change what we thought about planets and stars.

Using this scientific interpretation, we can conclude that a minimum amount of heavy elements must exist before planets, moons, and giant planets can form close to their parent stars.

A plausible but uncertain hypothesis is that planets and/or other rocky worlds are necessary for life. As a result, life in the Universe could not exist until enough heavy elements were present to form planets.

Elements here, elements there

The first stars that form are the first ones to produce the most abundant elements in the universe, other than hydrogen and helium: carbon, oxygen, nitrogen, neon, magnesium, silicon, and iron.

Although their content is no longer pristine, they can only increase the abundance of heavy elements by about 0.0001% over what we find in the Sun, so the next generation of stars will remain exceedingly devoid of them.

In order to form a rock-and-metal-rich planet, many generations of stars must process, reprocess, and recycle the detritus from each previous generation. As a result, Earth-like planets are impossible until a critical threshold of those heavy elements is reached.

Siegel explains it perfectly: For over half a billion years and possibly even longer, there will be no Earth-like planets forming at all.

Planets like Earth will eventually be found only in the richest, central regions of galaxies after several billion years. Several billion years later, Earth-like planets can be found in the central galactic region and portions of the galactic disk.

From then until now, there are still many areas, particularly in the outskirts of galaxies, in their galactic halo and in globular clusters spread all over the galaxy, in which heavy element-poor regions can’t form planets like Earth.

In other words

By extrapolating from the raw numbers, we found that there are at least as many planets as stars in the universe. While this statement remains true, it’s no longer a smart assumption to assume that every star in the universe has a planet.

Instead, as Siegel writes, planets are most abundant wherever heavy elements needed to form them via core accretion are plentiful, and planet numbers decline as parent stars contain fewer and fewer elements.

Whatever the case, and no matter how many stars there might be out there, and no matter how many of these stars have families of planets around them, we are still in our infancy of exploring the cosmos. In other words, it is very likely that all of the assumptions about the universe, galaxies, stars, and planets we have today will completely change in the future when we develop and build better telescopes and devices to look into the vastness of space.

The James Webb Telescope is just one example of how a piece of technology can completely change our views about the universe.

You can read more about what Siegel thinks in this article.


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Written by Ivan Petricevic

I've been writing passionately about ancient civilizations, history, alien life, and various other subjects for more than eight years. You may have seen me appear on Discovery Channel's What On Earth series, History Channel's Ancient Aliens, and Gaia's Ancient Civilizations among others.

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