Planet-finding Through gravity’s lens

There’s a planet out there, orbiting a star. There are, in fact, many of them out there. Thanks to studies made from ground-based observatories as well as such orbiting telescopes as HST, Kepler, TESS, and others, we know about thousands of them. We know they’re there. We know that some are hot Jupiters. Others are more Earth-type worlds. Some are gigantic ice giants like Neptune or Uranus. There are direct images of such worlds, but they’ really don’t show us surface characteristics. Those types of images may come someday. For now, we’re limited to detecting worlds and characterizing them based on spectral studies or direct imaging. That’s how we get knowledge about their orbital positions and information about their stars.

Why study these worlds? There’s a lot we are still learning about our own solar system. Its history and evolution are still a story to be filled in with data. Looking at other planetary systems gives us a look at our own in different stages of development. It’s a bit like looking at a forest and seeing trees in different stages of growth. There are stars and planetary systems in our neck of the galactic woods. So, there’s very likely a good example of planetary collections at all stages of life

The Lens of Gravity

Finding worlds around distant stars is hard. They’re small and dim, compared to their stars, which are large and bright. Astronomers use all kinds of techniques to spot them. To me, one of the more interesting ways is via gravitational lensing, which sounds weird but is perfectly natural. It needs an object with a strong gravitational pull to pass in front of a more distant object. Gravity warps space. Anything that moves through warped space is affected. So, a beam of light from a more distant object that passes through the warped gravitational space around a foreground object looks warped. In more practical terms. the image of the more distant object looks warped. That’s because the light’s path is warped like a funhouse mirror by the gravity of the closer object.

Lensing History

I first learned about gravitational lensing as it relates to looking at very distant objects, such as quasars. Their light passes through warped space, and the effect can be downright eerie. The Einstein Cross, for example, is the light from a single quasar. That light gets warped as it passes through the gravitational pull of a galaxy cluster closer to us than the quasar. The result: a “cross” made of four images of the single quasar. Those images are created by the lensing effect of the galaxy cluster.

So, if that works for more distant, bright objects, can it work for closer, smaller, dimmer objects? It turns out it can. And, astronomers have known this for quite a while.

A Small Planet Caught in a Lens

For example, a couple of years ago, an amateur astronomer spotted a gravitational lensing event that involved one star passing in front of another star. Now star-star lensing events are understandable but still rare. While there are a lot of stars in a galaxy, the chances of one passing in front of another from our point of view are still low. Space is big and not as “packed” as we’d like to think in most places. So, finding one of these is kind of an astronomical jackpot. Data from the event can give information about both stars.

Anyway, the observer, named Tadashi Kojima (from Japan), spotted this event and immediately sent out an alert. Pretty soon everybody was checking it out. That included astronomers at the big observatories. It turns out that the lensing event also showed that the foreground star has a planet in orbit. Astronomers from the University of Tokyo observed the event using telescopes around the world for more than three months. The data they collected about the star and its planet was fascinating. For one thing, the star is about half the mass of our Sun. Its planet is more massive than Neptune, but orbits at the same distance as Earth does from the Sun. It’s also a relatively close neighbor, lying about 1,600 light-years away. The more distant star was about a thousand light-years more distant.

What Does it Mean?

We are so accustomed to OUR Uranus and Neptune-type planets out in the distant reaches of the solar system. Sometimes that leads people to think that ALL solar systems have their giant planets out in the boonies. But, as planetary scientists study more of these exoplanet systems, they find giant planets in remarkably close orbits. That leads to questions about where ours formed in the early protoplanetary nebula. If they formed close in, then why are they now out in the solar system sticks? Studying more stars with giant planets close in can help answer those questions for our own solar system. This particular discovery also tells astronomers that more Neptune-sized planets orbit close to their stars at some point in their evolution.

So, gravitational lensing, which has been an observational tool for years now, is rooting itself firmly into the toolbox of planetary detection methods. It’s a perfectly useful way to peer at distant objects and get major data about them. Ultimately, it all comes back to help us understand how and why the universe works in ways both big and small.

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