Astronomers Debate: How Many Habitable Planets Does Each Sun-Like Star Have?
“We know that there are between 200 billion and 400 billion stars in the Milky Way galaxy. About 20% of those stars are Sun-like, for about 40-to-80 billion Sun-like stars in our galaxy. There are very likely billions of Earth-sized worlds orbiting those stars with the potential for the right conditions to have liquid water on their surfaces and being otherwise Earth-like, but whether that’s 1 or 2 billion or 50 or 100 billion is still unknown. Future planet-finding and exploring missions will need better answers than we presently have today, and that’s all the more reason to keep looking with every tool in our arsenal.”
Most of the time, in science, the quality of our data drives the size of our uncertainties. When we have very little data and it’s only of poor quality, our uncertainties tend to be large; when we have lots of very good data, our uncertainties shrink. NASA’s Kepler mission has provided astronomers with an unprecedented suite of data on exoplanets, revealing thousands of new worlds beyond our Solar System. And yet, despite all it’s found, if you ask the simple question of “how many Earth-like planets orbit a typical Sun-like star,” answers disagree by a factor of 100: from about 1% of stars have them to there’s between 1 and 2 for each and every such star.
What’s the real story? Where do these uncertainties arise, and are they larger than they need to be? Come get the full story (and watch David Kipping’s video at the end) and find out!
Ask Ethan: What Has TESS Accomplished In Its First Year Of Science Operations?
“When it’s a bright, sunny day and you want to see an object in the sky that’s very close to the Sun, what do you do? You hold up a finger (or your whole hand) and block out the Sun, and then look for the nearby object that’s much intrinsically fainter than the Sun. This is exactly what telescopes equipped with coronagraphs do.
With the next generation of telescopes, this will enable us to finally directly-image planets around the closest stars to us, but only if we know where, when, and how to look. This is exactly the type of information that astronomers are gaining from TESS. By the time the James Webb Space Telescope launches in 2021, TESS will have completed its first sweep of the entire sky, providing a rich suite of tantalizing targets suitable for direct imaging. Our first picture of an Earth-like world may well be close on the horizon. Thanks to TESS, we’ll know exactly where to look.”
NASA’s TESS has completed its first year of science operations, where it’s just finished surveying the entire southern celestial hemisphere. With 13 separate observations of 27 days apiece, it’s managed to find over 800 candidate planets, including some spectacular examples of planetary systems that are unlike any we’ve ever seen before.
But the real power of TESS is that it tells us where to look to directly image Earth-sized and super-Earth-sized worlds around stars beyond our own. Find out what we’ve learned, so far, today.
This Is Everything That’s Wrong With Our Definition Of ‘Planet’
“There are many people who would love to see Pluto regain its planetary status, and there’s a part of me that grew up with planetary Pluto that’s extraordinarily sympathetic to that perspective. But including Pluto as a planet necessarily results in a Solar System with far more than nine planets. Pluto is only the 8th largest non-planet in our Solar System, and is clearly a larger-than-average but otherwise typical member of the Kuiper belt. It will never be the 9th planet again.
But that’s not necessarily a bad thing. We may be headed towards a world where astronomers and planetary scientists work with very different definitions of what attains planethood, but we all study the same objects in the same Universe. Whatever we call objects — however we choose to classify them — makes them no less interesting or worthy of study. The cosmos simply exists as it is. It’s up to the very human endeavor of science to make sense of it all.”
Next month will mark 13 years since the International Astronomical Union (IAU) officially defined the term planet and ‘Plutoed’ our Solar System’s (up-until-that-point) 9th planet. With an additional 13 years of knowledge, understanding, data, and discoveries, though, did they get the decision right?
Certainly, there were aspects that needed to be revised, but the IAU’s definition comes along with some major gaps and mistakes. We can do better! Come learn how.
Starts With A Bang Podcast #45: Beyond Earth 2.0
With all the planets out there in the galaxy and Universe, it’s only a matter of time and data until we find another one with life on it. (Probably.) But while most of the searches have focused on finding the next Earth, sometimes called Earth 2.0, that’s very likely an overly restrictive way to look for life. Biosignatures, or more conservatively, bio-hints, might not only be plentiful on worlds very different from our own, but around Solar Systems other than our own. Earth-like worlds, in fact, might not even be the most ubiquitous places for life to arise in the Universe.
I’m happy to welcome scientist Adrian Lenardic onto the Starts With A Bang podcast, and explore what just might be out there if we look for life beyond our idea of Earth 2.0!
(Image credit: JPL-Caltech/NASA.)
Is Alien Life Hiding Beyond Earth 2.0?
“It may be the case that life is rare in the Universe, in which case it will require us to look at a lot of candidate planets — possibly with very high precision — in order to reveal a successful detection. But if we search exclusively for planets that have similar properties to Earth, and we restrict ourselves to looking at parent stars and solar systems that are similar to our own, we are doomed to get a biased representation of what’s out there.
You might think, in the search for extraterrestrial life, that more is more, and that the best way to find life beyond Earth is to look at greater numbers of candidate planets that might be the Earth 2.0 we’ve been dreaming of for so long. But non-Earth-like planets could be home to life that we’ve never considered, and we won’t know unless we look. More is more, but “different” is also more. We must be careful, as scientists, not to bias our findings before we’ve even truly started looking.”
If we want to find life elsewhere in the Universe, it only makes sense to look at our own world, where we know it was successful, and to try and find other worlds similar to our own. But that absolutely cannot be the only thing we do, or the only inhabited worlds we’ll even have a chance at finding are the ones similar to our own.
We have many good reasons to favor or disfavor the probability of finding life on a variety of planets and moons out there in our galaxy, but the real truth of the matter is we don’t know how common or rare life is. We don’t know how common or rare a huge variety of processes are, or even whether life is more likely to arise under conditions very different from those we find on our own planet.
It’s possible that Earth-like worlds are the best bet for life in the Universe, but don’t count out the non-Earth-like worlds without looking. Here’s what everyone hopeful for discovering alien life should keep in mind as we search the galaxy.
Ask Ethan: Can We Find Exoplanets With Exomoons Like Ours?
“But, by far, the best possibility we have today is through direct measurement of a transiting exomoon. If the planet that’s orbiting the star can make a viable transiting signal, then all it will take is the same serendipitous alignment to have its moon transit the star, and sufficiently good data to tease that signal out of the noise.
This is not a pipe dream, but something that has already occurred once. Based on data taken by NASA’s Kepler mission, the stellar system Kepler-1625 is of particular interest, with a transiting light curve that not only displayed the definitive evidence of a massive planet orbiting it, but of a planet that wasn’t transiting with the exact same frequency you’d expect orbit after orbit.”
If you want to find an exoplanet, the most successful methods are to look for the effect it has on the light from it’s parent star. But what about if you wanted to find an exomoon? There are some subtle effects at play, but if we think hard about what they might be, we can come up with a series of methods that could reveal an exomoon’s presence indirectly, and pinpoint exactly where and when we could look to try and detect one directly. Thought to be a great technique for the upcoming James Webb Space Telescope to take advantage of TESS data, we’ve actually succeeded once already, using the Hubble/Kepler combo!
You may have missed it, but we think we’ve found the first exomoon as of late last year. What does the future hold for exomoons? Find out on this week’s Ask Ethan!
Ask Ethan: What Will Our First Direct Image Of An Earth-Like Exoplanet Look Like?
“[W]hat kind of resolution can we expect? [A] few pixels only or some features visible?”
I’ve got good news and bad news. With the next generation of space-based and ground-based telescopes on the way, we’ll finally be able to image Earth-sized and super-Earth-sized planets around the nearest stars to us directly. Unfortunately, even the largest of these telescopes won’t be able to resolve these planets beyond being a single pixel (with light leaking into the adjacent pixels) in angular size. But even with that limitation, we should be able to recover signatures of continents, oceans, icecaps, clouds, atmospheric contents, water, and potentially even life.
Come find out what we will (and won’t) be able to do with our first direct images of Earth-sized exoplanets, coming to you in just a few years!
Incredible First Discoveries From NASA’s New Exoplanet-Hunting Spacecraft: TESS
“The ultimate goal of TESS is to find possible Earth-like worlds, and star systems which may house rocky, potentially habitable worlds. Because TESS is optimized to scour the stars nearest to us, it’s greatest finds will be among the first targets for future, more powerful observatories that can not only detect these worlds, but measure their atmospheric contents. If we get lucky, some of those worlds might house molecules like water, methane, carbon dioxide, or even oxygen in their atmospheres.
It won’t be a slam-dunk that these worlds are inhabited, but TESS takes us one step closer towards finding the nearest worlds that might be humanity’s greatest hope for finding life outside of our own Solar System. The worlds we’ve found so far are absolutely fascinating, and just a few months into its primary mission, TESS is easily meeting even the loftiest expectations for it. By time the James Webb Space Telescope launches, TESS should provide us with many worlds that just might be the best place to look to take our next great leap towards our ultimate goal: finding an inhabited world.”
NASA’s exoplanet-hunting satellite, TESS, was launched in April of 2018, began taking data in July, and released their first data to the world last month. That data contains around 300 candidate exoplanets, and the first eight of them have already been confirmed. From worlds so hot that they might have liquid rock on their surface to a solar system so strange we’ve never found anything like it, these are the first highlights.
Someday, TESS might lead us to our first world with signs of life on it. Here’s where we are so far.
Aliens? Or Alien Impostors? Finding Oxygen Might Not Mean Life, After All
“This doesn’t mean that finding an Earth-like world with an oxygen-rich atmosphere won’t be incredibly interesting; it absolutely will be. It doesn’t mean that finding organic molecules coincident with the oxygen won’t be compelling; it will be a finding worth getting excited over. It doesn’t even mean that it won’t be indicative of life; a world with oxygen and organic molecules may well be overflowing with living organisms. But it does mean that we have to be careful.
Historically, when we’ve looked to the skies for evidence of life beyond Earth, we’ve been biased by hope and what we know on Earth. Theories of dinosaurs on Venus or canals on Mars still linger in our memories, and we must be careful that extraterrestial oxygen signatures don’t lead us to falsely optimistic conclusions. We now know that both abiotic processes and life-dependent ones can create an oxygen-rich atmosphere.
The hard problem, then, will be disentangling the potential causes when we actually find our first oxygen-rich, Earth-like exoplanet. Our reward, if we’re successful, will be the knowledge of whether or not we’ve actually found life around another star.”
If you were looking for life around a planet orbiting another star, how would you do it? Your first inclination might be to look for something just like Earth: an Earth-mass planet with Earth’s size and Earth’s orbital parameters around a Sun-like star. You might then go a step further and try to examine its atmospheric contents. If you found a large amount of oxygen and organic molecules in the same atmosphere, you might conclude that you’d found it: a world beyond our Solar System that was inhabited. But that’s not necessarily the case!
Out of Dr. Sarah Hörst’s lab comes a new finding: oxygen and organics can arise through abiotic processes on exoplanets. Oxygen may not mean life, after all.
Why Haven’t Scientists Found ‘Earth 2.0’ Yet?
“Over the past 30 years, astronomers have gone from zero known extra-solar planets to thousands. Periodic changes in a star’s motion or regular brightness dips give them away. Thanks to these techniques, we’ve revealed the masses and radii of worlds nearby and thousands of light years away. Over 200 are Earth-sized, with many residing in the so-called habitable zone around their stars. Yet with everything we’ve found, there are no potentially habitable Earth-like worlds around Sun-like stars.”
One of the greatest success stories over the past 30 years is the giant leap forward we’ve taken in understanding what worlds lie beyond our Solar System. We’ve gone, in that time, from exactly zero known planets beyond our Solar System to thousands. We’ve found worlds far larger than Jupiter, some of which revolve at distances interior to even Mercury’s orbit. We’ve found planets around blue supergiants and red dwarfs. And we’ve discovered small worlds, some of which are even smaller than Earth. Some of them even occur in the so-called habitable zone of their stars.
Yet, despite all of this, we have yet to discover a single Earth-sized world at an Earth-like distance orbiting a Sun-like star. Here’s why we haven’t gotten there yet.