We get a lot of exciting science news about new exoplanets routinely discovered by powerful space telescopes — the planets that orbit stars other than our own Sun. But you might be surprised to know that the search for a new planets in our own solar system is ongoing. In fact, researchers have spent decades combing the night sky for evidence of other planets at the far end of our own solar system.

In February, researchers announced they had narrowed the search field for a hypothetical ninth planet beyond Neptune by nearly 80%, raising hopes that astronomers might soon solve this mystery. GBH's All Things Considered host Arun Rath spoke about the search for unidentified planets in our own solar system with Matthew Holman, an astrophysicist for the Smithsonian and lecturer at Harvard University.

Arun Rath: So first, tell us more about this Planet Nine hypothesis and the evidence that astronomers see for it.

Matthew Holman: Well, let me try to answer a couple of different questions. You asked specifically about Planet Nine, but I also want to address the question of: Are there any other planets — of any type — in our solar system? So, the evidence for that is really about the ongoing process of exploration of our solar system. As technology has advanced, every time there's been some leap in our technology, we find more things either beyond the fringe of our solar system as we knew it at the time, or we find even smaller things. Each time there's been a big leap, we find more things farther away. And, for example, we've found things that are comparable to or bigger than Pluto.

The volume of our solar system that we've been able to probe is really minuscule. So basically, if we think about how much volume there is to explore, there's every reason to believe that there could be large things out there that we just haven't been sensitive to before.

Rath: Obviously, against the backdrop of the vastness of space, it's kind of hard to get your head around.

Holman: Yeah, and as you move something farther away from the sun, things appear fainter. We're looking at reflected sunlight, and the sunlight diminishes at the square of the distance. So if you move something twice as far away, it's four times fainter. Then that light has to reflect and come back and it's another factor of four times fainter. So if you were to move something twice as far away, it's 16 times fainter.

That kind of gives you the sense of the challenge that really exploring the bigger distances in our solar system requires big advances in telescopes and other technology.

Rath: It's wild how it seems like it's easier to find exoplanets with other stars, because we have the other star behind them, than it is to find these objects on the outer edge of our own solar system.

Holman: Yeah, that's a great point. We've found thousands of exoplanets and we have found thousands of things beyond Neptune at this point. But, it's far more challenging to find even more of these distant members of our solar system.

Rath: So what is the evidence that we have that there might be an additional planet or more?

Holman: That's this specific Planet Nine hypothesis, and there are a few lines of evidence. So we call the things beyond Neptune “trans-Neptunian objects,” or TNOs, and there are lots of different types of them. Some of them have circular orbits that are not much farther beyond Neptune.

To give you a sense of scale, the distance from the sun to the Earth is what we call one astronomical unit. Neptune is at 30, so it's way out there. And the trans-Neptunian region, where we know of a lot of objects, is between 35 and 50 astronomical units. So there's a bunch of stuff there that we have found and taken a good census of.

But then there are some things that are much farther away, and some of these we call extreme TNOs. What's really peculiar about those is that they never come that close to Neptune. They don't come close enough to Neptune for Neptune to have much of a gravitational influence on them. In fact, we don't really know how those objects originated. By that I mean, if we take what we know about the current planets — if we started with things scattering among the giant planets like we think happened in the late stages of planet formation — would we end up with some of these really distant objects? The answer is no.

That means that something was either different in the late stages of the planet formation, or we're not accounting for all the gravitational effects with the planets as we know them. If there was another planet, that could explain those distant objects. That's just one component of the evidence.

Of these extreme TNOs — they have these long elliptical orbits. One of the peculiarities is that they seem to be lined up, not precisely, but if you look out on the sky, they seem to be clustered and in a particular direction. On top of that, they seem to be close to a plane, but that's not quite the plane of our solar system. That's really odd to see that. Maintaining that kind of a configuration requires something like an additional planet.

Those are the big lines of evidence. That's where the argument begins as to whether that's evidence, or is this the result of something we call observational bias. Meaning, do we see this clustering because that's the only place we've looked? Or is it: we've looked everywhere, and this is where we find it?

Rath: Super interesting as we're coming up against the edges of our knowledge on this. Can you compare this search to the search for the last couple of planets that were found, like Neptune and Uranus?

Holman: The way that Uranus was discovered was with a backyard telescope. I mean, it was advanced technology at the time, but was just discovered from somebody's garden looking out and saying, "Hey, that's something that I haven't seen before." Neptune has a more interesting story in that, after Uranus was discovered and its orbit was tracked over the course of years, people began to see deviations from where it was predicted to be. Those deviations suggested to people like Urbain Le Verrier that there might be something else tugging on Uranus.

They made specific predictions of another body of a certain mass in a certain position on the sky. People used that prediction and went out and looked and found what is now Neptune. So what is similar about the Planet Nine hypothesis is that we're not really seeing deviations in the orbits of specific objects — but we do see patterns in the orbits of a collection of other objects that are the result of the gravity of another body. So there's a similarity.

But in terms of the technology that's dramatically different. The telescopes are much, much bigger. Not only are there cameras, they're digital cameras. So we're not just using the human eye to see these things. We're using very sensitive instruments that can look at very large regions of the sky at once. So it's possible to survey essentially the whole sky, systematically.

Rath: I just love this. It's so fascinating. Everything that we don't know and the weirdness about our own celestial backyard. Thank you so much for explaining this to us.

Holman: It's been my pleasure.