Can We Find Exoplanets Using Gravitational Waves?

Is it possible to find exoplanets using their gravitational wave signatures? If the orbit of the planet is close enough and short enough to the host star, then yes it is. But boy what a system it would need to be!

Published on 11th Aug, 2018

Hello Space Fans and welcome to another edition of Space Fan News. In this episode, is it possible to find exoplanets using their gravitational wave signatures? It turns out it is possible if the orbit of the planet is close enough and short enough to the host star, in other words, if the gravitational luminosity (I love that term) is bright enough. So what kind of exoplanets can we expect to find using gravitational waves and from where? Stick around to find out. By now we’ve all become familiar with the fact that we have now entered the era of gravitational wave astronomy, which has opened up and entirely new window into the cosmos for humanity. Using the Laser Interferometer Ground-based Observatory, or LIGO, we can see the gravitational waves created when two massive objects collide. LIGO is our first instrument capable of seeing these gravitational wave events. It does this by placing a set of mirrors far enough apart from each other that when a gravitational wave passes by, the mirrors move a very tiny amount and causes light from a very accurate laser being bounced off those mirrors to interfere with itself. From those interference patterns, it is possible to estimate the size of the objects that collided and get a very rough idea of the location in the sky. Basically, LIGO feels the gravitational wave and the interferometer mirrors literally ride that wave until it passes. But LIGO is very limited in the kinds of events it can detect, but first, let’s all learn a new term: gravitational luminosity. The authors of this paper I’m about to tell you about used this term which I’ve never heard before. Gravitational Luminosity is be analogous to electromagnetic radiation luminosity, only with gravitational waves. The larger the gravitational wave signal, the brighter the gravitational luminosity. Cool. Right now, LIGO can only see very bright gravitational wave eruptions. Those with very high gravitational luminosity. It’s pretty much limited to seeing large black hole and nearby neutron star mergers. There have been seven events detected so far by LIGO and while some are very far away, the most distant confirmed event was about 3 billion light years away, and all of them have been from merging black holes tens of times the mass of the Sun, or neutron star mergers. So while LIGO is giving us our first peek into the universe using gravitational waves, astronomers are starting to wonder what the future might be like using this new wavelength available to humanity. Whenever two massive objects interact by either merging or just spinning fast around each other, gravitational waves are emitted as a result, and their gravitational luminosity is determined by the mass of the objects and their proximity to each other. Which brings me to the authors of a paper that I found interesting on Astro-ph who did the math and tried to determine if we could find exoplanets around other stars in our galaxy using gravitational waves. The answer is that yes, we should be able to see them, but not with LIGO and not just any old exoplanet, it is nowhere sensitive enough. We’d need at least an instrument like LISA, the space-based Laser Interferometer Space Antenna, a successor to LIGO which with have three mirrors spaced tens of thousands of kilometers apart and would allow us to see much fainter events with lower gravitational luminosity. And with LISA, we’d only be able to detect what are called Ultra short period exoplanets. We’re talking about orbits on the order of, oh, only an hour or so! An hour! That would be the planet’s year folks. And the planet would need to be relatively large, on the scale of a Hot Jupiter. It stands to reason that the largest planets going around the star this close and this fast to the star would be the brightest, but, as you can imagine, compared to black hole mergers, the gravitational luminosity of a large planet whipping around the host star every hour or so would be quite weak. But it isn’t impossible to see. And the system would need to be close, because the closer, the brighter. So, according to these guys, we should be able to detect large super jupes whipping around its star once an hour that aren’t too far away. Super jupes, another new term today, remember you heard it here first folks. It looks like there are a good number of these ultra short period super jupes around. In 2016, astronomers published a paper saying that in the Kepler data set, they had found 19 candidate exoplanets with orbits of less than a day and the authors of this study used 14 systems from the database to base their calculations on, so there are examples of these types of systems out there. The authors state that among the 14 examples they calculated, three of the systems have gravitational luminosity well within the sensitivity of a LISA-class gravitational wave observatory. What’s more, and I found this amazing, is that the gravitational luminosity can be on par with, and even greater than the electromagnetic luminosity of the star! Think about it, the gravitational wave brightness can approach, and even surpass, the electromagnetic brightness. Remember those three systems I told you about that were promising? One of them, J1433 b, has a regular EM brightness, this is what you’d see from a telescope, of 1.18 times ten to the thirty ergs per second. That’s about three ten-thousandths as bright as the Sun so it’s very faint. But it has a gravitational luminosity over twice that: 2.69 times ten to the thirty ergs per second. Rule of thumb: one erg has the energy of an ant push-up. If you could imagine little ants doing a workout... What this means is that in systems where the gravitational brightness is equal to or greater than the regular electromagnetic brightness, this interaction could have an effect on the habitable zone of the system. The gravitational waves coming from a planet whipping around a star once an hour or so could literally shake up the system, any other planets in the system would be getting seasick riding all those gravitational waves, and I can see how it would have some implications for the habitable zone. The authors of the paper didn’t really go into that very much, but it’s an interesting thought experiment, isn’t it? How would the habitable zone of a star be affected by high levels of gravitational wave luminosity? Let me know your ideas in the comments below. Well that’s it for this episode Space Fans, we learned a new term today - Gravitational Luminosity, and we learned that it’s possible to find certain kinds of exoplanets using gravitational waves. You deserve a new t-shirt. I just got this new one from Michal at, if you like it, please pick one up at the url in the description box, you’re helping deep astronomy and a dedicated space fan. Thanks to all Deep Astronomy Patreon supporters who keep the ads off on all Deep Astronomy postings, thanks to all of you for watching, and as always, Keep Looking Up! Read Paper here: Kepler study here: Get a cool Deep Astronomy T-shirt here: Consider supporting Space Fan News: to ensure you get current space & astronomy news each week! 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