# Can exoplanets be found using neutrino detectors?

I read that KamLAND can detect geoneutrinos produced by thorium and uranium decay in Earth's crust.

Could a larger detector detect neutrinos from other planets in the solar system or perhaps even exoplanets?

• Wouldn't it be swamped by the neutrinos coming from the star the planet was orbiting? Jun 29 '20 at 21:57

## 3 Answers

The answer is presumably no.

Let's assume that the nearest possible exoplanet is in the Proxima Centauri system, and it presents an electron antineutrino luminosity identical to that of Earth. The flux from the exoplanet at the KamLAND detector would then be weaker than the flux from Earth by a factor of roughly $$\left(\frac{\text{Radius of Earth}}{4\;\text{light-years}}\right)^2\approx2\times10^{-20}$$ thanks to the inverse-square law. So the intrinsic signal strength is likely to be essentially zero.

More concretely: the KamLAND collaboration's 2013 measurement of the Earth's geoneutrino flux was approximately $$\sim3.4\times10^6\;\text{cm}^{-2}\text{s}^{-1}$$, which isn't a lot considering the low probability that a given neutrino will interact with the detector. Indeed, the group reported only $$116^{+28}_{-27}$$ events in their period of monitoring - and some of that was during a time where nearby nuclear reactors were off and therefore not producing neutrino noise!

Any signal from an exoplanet would be tiny and easily lost in the background of all of the other possible neutrino sources.

• FWIW, according to en.wikipedia.org/wiki/Neutrino_detector "the only confirmed extraterrestrial sources so far as of 2018 are the Sun and the supernova 1987A in the nearby Large Magellenic Cloud". Jun 29 '20 at 10:53
• Quick check: which is bigger, 2^30 or 10^20, because I've heard of 2^-30 scale measurements being made. Jun 29 '20 at 22:20
• @Joshua 2^30 =~ 10^9 Jun 29 '20 at 22:36
• @Joshua: Rule of thumb: 2^10 is about 10^3, so 2^30 is about (10^3)^3 = 10^9. You can do this with most large powers of two. Jun 30 '20 at 4:22
• @Joshua Also that scale would likely be with respect to the strongest source, i.e. solar neutrinos, of which there are even a few orders of magnitude more than geoneutrinos. And with regards to that, even if we had sensitive enough instruments, they would also need an extremely precise angular resolution, otherwise exoplanet neutrinos would probably be drowned out by the neutrinos sent out by its host star.
– mlk
Jun 30 '20 at 8:06

Other answers are good at explaining that we are bad at detecting neutrinos - or that neutrinos are bad at being detected. However, it should be also be pointed the differences between neutrinos and electromagnetic waves that make the later a lot more useful than the former to observe distant objects.

There are two main differences:

• Neutrinos nearly don't interact with matter, while electromagnetic waves interact with matter in some rather simple ways - from an engineering point of view - so we can build on that to detect very faint electromagnetic signals. Obstacles don't stop neutrinos, but detectors neither. Then, we only can detect very large quantities of neutrinos - like that from the Sun - with very large detectors. Furthermore, since neutrinos nearly don't interact with anything, they don't interact with exoplanets and even if we could detect them better, they would carry very little information about any exoplanet.
• Electromagnetic waves can be blocked and focused. That allows us to tell apart very faint sources from the more powerful ones. At night we can see distant stars even with naked eye, because the Sun's light is blocked and because we can resolve small points. Neutrinos are not blocked by anything and we nearly can't tell apart them by direction. Therefore, even if we were better at detecting them, the very small signal from stars would be masked by the stronger one from the sun. In fact, doing astronomy with a neutrino detector would be like doing astronomy with an omnidirectional photometer on daylight.
• Neutrino detection isn't always omnidirectional. Some of the detection methods listed at en.wikipedia.org/wiki/Neutrino_detector have some direction sensitivity, especially for very high energy (GeV) neutrinos. Jun 30 '20 at 15:36
• I've added "nearly". It's true that some kind of detectors give information about direction, but they are still far away from resolving small sources when compared with visible light. Since the question is about detecting exoplanets, I let the answer as it is, for simplicity and because the resolutions needed are a lot of orders of magnitude smaller than achievable.
– Pere
Jun 30 '20 at 19:26
• Understood. I just wanted to mention that direction detection isn't totally hopeless, but it does require the neutrinos (or antineutrinos) to have enough energy & momentum. But certainly, building a neutrino telescope that can actually produce useful images from neutrinos seems very unlikely, since it's very difficult to refract or reflect them. Jun 30 '20 at 20:27

After a great deal of scientific and technology advance (few decades or more) , ... maybe. Neutrinos are very good at penetrating obscuring matter that the light cannot. If we can see them, they may reveal a lot of hidden objects. Neutrinos are also pretty good evading detection, for now.

Just like the astronomy observations gradually expanded from the visible light to weak visible light (telescopes), near infrared and UV (chemical photography), radio waves (radio telescopes), infrared, far UV, X-ray, gamma-ray (space telescopes), gravity waves (LIGO and friends), etc, ... one day we may as well have high-resolution neutrino pictures that can either resolve planets, or reveal their presence by other means.

@HDE226868 did a good calculation about modern detectors. Looks like no faintest hope, right now. We are pretty poor at detecting neutrinos and can only deduce their approximate direction if they have a high enough energy (see Ice Cube).

• It's not that we're bad at detecting them, it's that there is no obvious law of physics which would enable you to build a better detector. The only reason we can detect them at all is because the sun emits scads of them continuously. Jun 30 '20 at 4:25
• Still boils down to us being bad at detecting them. The Standard model does a good job, but it is "just a theory" :) . It can be extended, expanded, reworked, etc... in the future. Jun 30 '20 at 8:06
• Just because the Standard Model is an incomplete description of how the universe works, you don't get to invent fantasy physics that contradict everything we know about how neutrinos interact with matter.
– user24157
Jul 1 '20 at 13:28