Category Archives: astronomy

astronomy

Upping the Number of Habitable Worlds

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Last week, the SETI Institute put out a study that claimed there could be as many as 300 million potentially habitable worlds in our galaxy. That’s a pretty good number, considering that nearly every star is likely to have (or have had) at least one planet (according to another study). What’s more, given the distribution of stars and planets in the Milky Way, some of those potentially habitable worlds could be fairly close to us. This isn’t just some “pie in the sky” guesstimate. Scientist Jeff Coughlin (of the SETI Institute) and a team of researchers used Kepler Space Telescope data as well as observation sets from the European Space Agency’s GAIA mission to come to their conclusions.

A possibly habitable world, among the 300 million that scientists at SETI institute think exist in the Milky Way Galaxy.
An artist’s conception of Kepler-186f. It’s the first confirmed Earth-size planet found orbiting a distant star in its habitable zone. Credit: NASA Ames/JPL-Caltech/T. Pyle

Estimating the Number of Such Worlds

So, if there ARE 300 million potentially habitable worlds “out there”, what might they be like? Anybody who reads science fiction probably has a good idea. Visiting other planets is a staple of that genre. For those of us who read SF, the existence of worlds with life on them is not a new idea. But, for science to work, there has to be observational data of those worlds. That’s to feed any theories about what these places might be like. To answer the question, and come up with their estimate of 300 million, the SETI team devised a plan.

First, they studied other worlds that are similar in size to Earth. They looked for probably rocky worlds orbiting stars similar to the Sun. (SO, this cuts out rogue planets, which I discussed in my last entry.) And, of course, they wanted planets in the “habitable zone”. That’s where liquid water could exist on the surface of planets orbiting within it. Add in other factors, like how much light hits the planet from its star, and that gives scientists a good set of parameters for estimating how many such worlds exist. But, it still doesn’t tell us what they’re like.

Finding Life on Habitable Worlds

Determining life on distant worlds is not something we can easily figure out from Earth. Well, astronomers can use special techniques to study the atmospheres of those worlds. There are some chemical signatures in atmospheres that would indicate the presence of life. And, of course, one could direct the radio telescopes of the Allen Telescope Array, for example, in their direction, to see if any intelligent civilizations are beaming out signals.

That’s one way beings “out there” could tell if our planet has life, by the way. We’ve been sending signals to space for around a hundred years, from radio and TV broadcasts. They are racing out away from us at the speed of light, and there ARE some potentially habitable worlds within that signal path. (It’s actually an expanding balloon of signals called the “radio sphere”.)

Another way might be to look at our atmosphere as our planet transits the Sun. A distant observer, with direct line of sight to see such an event, would see sunlight streaming through our atmosphere. The gases in our air blanket would absorb specific wavelengths of light. That creates “dropouts” in the solar spectrum. Each element and chemical compound has a specific “fingerprint” that could show up in the spectrum. Those that life emits would clue in a distant researcher that there’s life on our planet.

The Search is Afoot

That is, in fact, what researchers plan to do: study the light from the star as it passes through a nearby planetary atmosphere. If the fingerprints of chemicals related to biotic activity are there, then it’s a likely sign that life exists there. Of course, it’s painstaking work. It isn’t done overnight. It requires detailed observations and a LOT of analysis. That’s the way the science works and such observations are happening.

They will take a while, so while we wait for that first momentous discovery, we can still marvel at the fact that there are 300 million planets out there to study. Some of them just HAVE to have life. We can’t be the only ones in the galaxy looking out and wondering who else is out there. Or, to put it another way, in his book with Ann Druyan, the late Carl Sagan wrote, “The universe is a pretty big place. If it’s just us, it’s just us, seems like an awful waste of space.”

astronomy, planets

Life on a Rogue Planet…. Or, Not

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I got to thinking about all the planets in the galaxy that we know about (or are about to discover). Are there weird ones that aren’t quite in the same category as the Earth-types, hot Jupiters, super Neptunes, etc?

Well, sure. There are rogue planets. They’re little worlds (usually smaller than Earth) that, end up wandering the galaxy instead of marching around stars.

For one reason or another, these “rogue planets” escaped their systems at some point during the formation process. It happens as worlds are jostled around in the disk of material from which they form. Most of the time, the jostling results in a reshuffling of worlds. This is likely what happened with our gas giant planets early in the solar system’s birth process. Gravitational interactions basically push things around. A planet born in one area moves to a more distant orbit. Or, in the case of a rogue planet, it is ejected from the system altogether. In some cases, they have never been formally gravitationally bound to a star. No matter their origins, these lonely rogues are doomed to wander the galaxy forever.

A free-floating rogue planet transits across a distant star, and the light from the star is microlensed by the planet. Many of these rogue planets exist in the galaxy.
An artist’s impression of a gravitational microlensing event by a free-floating planet.
Credit: Jan Skowron / Astronomical Observatory, University of Warsaw

Finding Rogue Planets

How do these planets get detected? About 4,000 know rogues exist in the galaxy. But, they’re not easy to see. Most are thought to be smaller than Earth. Since they’re not around stars, they don’t reflect light. But, their presence is detectable when their paths take them between us and a star. If the alignment is just right, they block a little light from the star. That short-term dimming during such a transit can be detected. (Measuring these short-term brightness flickers is, in fact, what the Kepler Space Telescope did, before it went out of commission.)

A long-term survey of stars, called OGLE (which the Optical Gravitational Lensing Experiment), spots such transits for lensing events. Lensing happens when a massive object, such as a star passes between Earth and a more distant object. The light from the more distant one gets ‘lensed’ by the gravitational influence of the larger one. That changes the brightness of the more distant object a bit.

The lensing objects don’t have to be stars. A planetary system can be involved. So can a rogue planet. And OGLE has been studying thousands of stars to find these microlensing events. In some cases, rogue planets are involved.

In the future, other telescopes, such as the upcoming Nancy Grace Roman Space Telescope will be able to measure these things from space. On Earth, a few ground-based observatories can also do broad surveys to look for microlensing, and thus find more rogue planets.

Could These “Free-floating” Planets Support Life?

The hunt for any planets in the galaxy (and beyond) has, at its heart, the search for other places where life could exist. Most astrobiologists agree that the places with the BEST chances for life are on planets orbiting stars. In particular, they should be in or very near the habitable zone around the star. That’s where liquid water could exist on a planetary surface. You also need light and warmth (which the star provides), and sources of food.

I really doubt that a rogue planet is a very friendly habitat for life. For one thing, there’s no warmth, and may not be any atmosphere or liquid water on its surface. Plus, since it got flung out from its star early in its evolution, it probably didn’t have time to entertain the formation of life.

Astronomers estimate there could be billions of these lonely worlds, floating between stars. So, with such a large number, there could be variations on the rogue planet theme. That’s why I’d never say “never” to any life. It’s possible, I suppose, that some form of microbial life could have gotten started before the planet was kicked out. So, there might be evidence of that.

Other Uses for Rogue Planets

I could certainly think of plenty of sci-fi scenarios where rogue planets could be useful in a galactic empire sense. I mean,what a great place (if you had the technology to do so) to stash a rebel base. Or a secret hideout. The possibilities are endless. But, any techno-savvy group is going to have to provide power, technology, and livable spaces for these places. That’s going to cost a lot. And, I would have to wonder about the return on investment. Still, it’s an intriguing idea, and until we get more data on these worlds, it’s fun to imagine such alternate timeline stories about them.