We haven’t found anything like the solar system. Is it a freak in space?

Since the historical discovery in 1992 Two planets orbiting a star outside our solar systemAnd the Thousands of new worlds They have been added to the rapidly growing list of “exoplanets”. in the Milky Way.

We learned many things from this A vast catalog of alien worlds orbiting alien stars. But there is one small detail that sticks out like a sore thumb. We haven’t found anything else like our own solar system.

This has led some to conclude that our parent star and its bosom could be outliers in some way – perhaps the only planetary system of its kind.

By extension, this could mean that life itself is an anomaly; The conditions that shaped the Earth and its crust of self-reproducing chemistry are difficult to replicate.

If you just look at the numbers, the outlook is bleak. By a large margin, the most numerous exoplanets we’ve identified so far are of a type not known to be conducive to life: giants and sub-planets, of the gas and possibly ice variety.

Most of the exoplanets we’ve seen so far orbit their stars very closely, practically hugging them; So close that the sizzling temperatures would be well above their known habitable range.

Artist’s impression of Superheater Jupiter star crossing. (ESO/M. Kornmesser)

It’s likely that as we continue to search, the stats will balance out and we’ll see more places that remind us of our own backyard. But the issue is much more complex than just looking at the numbers. Exoplanet science is limited by the capabilities of our technology. More than that, our impression of the true variety of alien worlds risks limiting our imagination.

What’s really out there in the Milky Way and beyond may be very different from what we actually see.

expectations and how to thwart them

Exoplanet science has a history of subverting expectations, right from the start.

“If you go back to that world you grew up in as a kid, we only know of one planetary system,” Planetary scientist Jonty Horner of the University of Southern Queensland told ScienceAlert.

And so it was this kind of implicit assumption, sometimes an explicit assumption, that all planetary systems are going to be like this. You know, you’ll have rocky planets near the star that’s very small, and you’ll have gas giants a long way from the star that was very big. And that’s what planetary systems will be like.”

Because of this, it took scientists some time to identify an exoplanet orbiting a main sequence star, like our sun. Assuming that other solar systems are similar to ours, the telltale signs of heavyweight planets dragging on their stars would take years to spot, just as it takes years for our gas giants to complete their orbit.

Based on such long periods of a single measurement, it does not seem worth the trouble to sift through a relatively short history of observations of so many stars to definitively examine the fellow solar system on the main sequence.

When they finally looked, the exoplanet they found was nothing like what they’d expected: a gas giant half the mass (and twice the size) of Jupiter orbiting very close to its host star, an age of 4.2 days, and an atmosphere scorching at temperatures of about 1,000 degrees Celsius (1,800 C). degrees Fahrenheit).

Since then, we’ve learned that these “Hot Jupiter”-type planets aren’t exotic at all. If anything, they seem relatively common.

We now know that there is much more diversity in the galaxy than what we see in our home system. However, it is important not to assume that what we can currently detect is all that the Milky Way has to offer. If there is anything like our solar system, it is very likely that it is beyond our detection capabilities.

“It’s very difficult for us to find things like the solar system, they’re a little bit beyond us technologically right now,” Horner says.

“It is very unlikely that terrestrial planets will be captured from any of the surveys we have conducted so far. It is unlikely that you will be able to find Mercury, Venus, Earth and Mars around a star like the Sun.”

How do you find a planet?

Let’s be absolutely clear: the methods we use to discover exoplanets are very clever. There are currently two working units of the exoplanet discovery kit: the transit method and the radial velocity method.

Either way, you need a telescope that is sensitive to very subtle changes in a star’s light. However, the signals each person is looking for couldn’t be more different.

For the transit method, you’ll need a telescope that can keep the star fixed in its view for a long period of time. That’s why instruments like NASA’s Transiting Exoplanet Survey Satellite (TESS) are such a powerful center that they can hold onto a portion of the sky for more than 27 days without being interrupted by Earth’s rotation.

The goal with these types of telescopes is to identify a transit signal — when an exoplanet passes between us and its host star, like a small cloud blocking some of the sun’s rays. These dips in light are small, as you can imagine. One flash is not enough to infer with confidence the existence of an exoplanet; There are many things that can dim a star’s light, and many of them are one-time events. Multiple transits, especially those that show regular periodicity, are considered the gold standard.

Therefore, larger exoplanets located at shorter orbital periods, closer to their stars than Mercury is to the Sun (some much closer, in orbits less than one Earth week), are preferred in the data.

The radial velocity method detects the oscillation of a star caused by the gravity of an exoplanet as it oscillates in its orbit. The planetary system, you see, doesn’t really revolve around a star, so much as it dances in a coordinated shuffling. The star and the planets revolve around a common center of gravity known as the center of gravity. For the solar system, this is a point very, very close to the surface of the sun, or just outside it, mainly due to the influence of Jupiterwhich is more than twice the mass of the rest of the planets combined.

Unlike a transit flash event, a shift in a star’s position is a continuous change that does not require constant monitoring to notice. We can detect the motion of distant stars orbiting their baryonic centers because this motion changes their light due to something called the Doppler effect.

As the star moves toward us, the light waves coming in our direction are compressed slightly, toward the bluer end of the spectrum; As you move away, the waves extend toward the red end. A regular “wobble” in the star’s light indicates the presence of an orbital companion.

Again, the data tends to favor larger planets that have a stronger gravitational influence, on shorter orbits and closer to their star.

Aside from these two notable methods, it is sometimes possible to directly image an exoplanet as it orbits its star. Although it is very difficult to do, it may become more common in the JWST era.

According to astronomer Daniel Baylis of the University of Warwick in the UK, this approach would reveal an almost opposite class From an exoplanet to a short orbit variety. In order to see an exoplanet without being overwhelmed by the glare of its parent star, the two objects would need a very wide separation. This means that the direct imaging method favors planets in relatively long orbits.

However, larger exoplanets can still be easily spotted through this method for obvious reasons.

“Each detection method has its own biases,” Bayliss explains.

He adds that the Earth, with a one-year ring around the Sun, is located between two ends of an orbital orbit favored by various detection techniques, so “it is still very difficult to find planets with a one-year orbit.”

What’s there?

By far the most numerous group of exoplanets is a class that is not even represented in the solar system. This is Minor Neptune – gas-covered exoplanets that are smaller than Neptune and larger than Earth.

A rocky planet surrounded by a purple haze and a star in the distance on the left
Illustration of mini Neptune TOI 560.01, which orbits its solitary star. (WM Keck Observatory / Adam Makarenko)

Most confirmed exoplanets are in orbits much shorter than Earth’s; In fact, more than half of them have orbits of less than 20 days.

We found most of the exoplanets orbiting single stars, like our sun. Less than 10 percent in multi-star systems. after mMost stars in the Milky Way are members of multiple star systems, with estimates as high as 80 percent in partnership orbiting at least one other star.

Think about that for a moment. Does this mean that exoplanets are more common around single stars — or that exoplanets around multiple stars are harder to detect? The presence of more than one source of light can distort or mask the very similar (but much smaller) signals we’re trying to detect from exoplanets, but it could also be a reason why multi-star systems complicate planet formation in some way.

And that brings us back home again, to our own solar system. As strange as the house sounds in the context of everything we found, it may not be at all uncommon.

“I think it’s fair to say that there are actually very common types of planets missing from our solar system,” Bayliss says.

“Superplanets that are a bit like Earth but twice the radius, and we don’t have anything like that. We don’t have these little Neptunes. So I think it’s fair to say that there are some very common planets that we don’t see in our solar system.

“Now, whether or not that makes our solar system scarce, I guess I wouldn’t go that far. Because there might be a lot of other stars out there that have a bunch of solar system-type planets that we haven’t seen yet.”

An artist's drawing of several planets and stars in the Milky Way.
This artist’s illustration gives an impression of how common planets are around stars in the Milky Way. (ESO/M. Kornmesser)

About to find out

The first exoplanets were discovered just 30 years ago and they orbit a pulsar, a star very different from our own. Since then, technology has improved out of sight. Now that scientists know what to look for, they can devise better and better ways to find it around a greater variety of stars.

And as technology advances, so does our ability to find smaller and smaller worlds.

This means that exoplanet science may be on the verge of discovering thousands of worlds hidden from our current view. As Horner points out, in astronomy, there are more small things than big things.

Red dwarf stars are a perfect example of this. They’re the most common type of star in the Milky Way – and they’re very small, down to about half the mass of the Sun. They are so small and dim that we cannot see them with the naked eye, yet they account for up to 75% of all stars in the galaxy.

Right now, when it comes to statistically understanding exoplanets, we’re working with incomplete information, because there are types of worlds we can’t see.

This is bound to change.

“I have this nagging feeling that if you went back in 20 years you would look at those statements that mini-Neptunes are the most common type of planet with just as much skepticism as if you looked back at statements from the early 1990s she said you You’ll only get rocky planets next to the star,” Horner told ScienceAlert.

“Now, I can be proven wrong. That’s how science works. But my thinking is that when we get to the point where we can discover things the size of Earth and smaller, we’ll find that there are more things the size of Earth and smaller than there are things the size of Neptune.” .”

And perhaps we will find that our whimsical little planetary system, with all its quirks and wonders, is not alone in the universe after all.


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