Are We on the Verge of Discovering an Earth-Like Exoplanet?

I announced awhile ago that I was on episode 347 of the Canadian, “The Reality Check” poscast where I talked about exoplanets and some hype — deserved or otherwise — about almost but never quite yet discovering Earth-sized exoplanets.

While they post a lot of links and other things on their website, they don’t post transcripts of what we actually talk about. Since I spent a solid many minutes writing and editing my segment’s text, I thought I’d post it here:

There’s lots of ways to talk about exoplanets, but I’m going to take the traditional approach and start with a very broad but brief overview of how we have found the few-thousand known extra-solar planets, or “exoplanets” for short. There are five main ways.

The most obvious is the most difficult: Direct Imaging. This is where you take your telescope and would look at a star and see the planet around it. This is almost impossible with current technology, and we have less than 20 exoplanets found this way. It’s so hard because the star is so bright relative to the planet and because most star systems are so far away. And obviously, if the planet is larger and farther away from the star, it’ll be easier to see.

The second main method has also only produced about 20 planets so far: Gravitational Microlensing. Einstein showed that large masses bend light, and we can see this in space when an object that’s far away passes behind a massive object that’s a lot closer. The light from the background object gets distorted and magnified, much like a lens … a lens caused by gravity. If the foreground object happens to be a star, and that star has a planet, then that planet can make a detectable contribution to the lensing, not only in amount, but in the exact shape of the lensing effect.

The earliest actual successful method was a special form of what’s called the Timing Method, specifically in this case, pulsar timing. Pulsars are incredibly dense stars called neutron stars, and we get a blast of radio waves every time one of its poles sweeps in the direction of Earth. These are so regular that any tiny perturbation can be detected and attributed to something weird, like a tiny planet tugging on it and so changing that regular spinning signal.

This is the same concept as the highly successful method that found the most exoplanets until a few years ago: Radial Velocity. The idea is that we normally think of a planet, like Earth, orbiting the sun. But it doesn’t really. It *and* the sun orbit a mutual gravitational point called the “barycenter” that is between the two. For Earth and the sun, that point is VERY close to the sun’s center, but it’s not quite in the center. That means that over the course of a year, as Earth goes around that point, the sun will, too (on the opposite side of that point). So, it will wobble very very slightly as it orbits the barycenter.

We can’t possibly observe this tiny tiny motion of other stars. BUT, we can use the light that star emits to do it by using the Doppler shift. That’s the phenomenon where if something is moving towards you, the waves it emits become compressed, and if it’s moving away from you, the waves get stretched out. The common example is a train whistle going from high to low pitch, but in astronomy, this is where the light is shifted to blue and then to red.

So, if the planet around another star is at its closest point to us, the star emits light and we see it all normal. As the planet starts to move away from us, the star starts to move very slightly toward Earth, and so its light will be very slightly blue-shifted. Then, the planet gets to its farthest point, and starts to move towards Earth, which means the star starts to move away, and we see its light red-shifted. This is an incredibly tiny effect, and the smaller the planet, the smaller the shift in the light. Or the pulsar timing change.

There was a lot of progress throughout the late 1990s and early 2000s in very high-resolution spectroscopy in order to get better and better at observing smaller and smaller planets. The easiest ones to observe are the largest because they make the biggest shift in the star’s light, and ones that are closest to their star are easier because you don’t have to observe as long. To observe a planet that has a 10-day orbit, you just have to observe that star for about a month from Earth to get decent statistics.

That’s why all the exoplanets discovered early on were what are called “Hot Jupiters,” since they were very large and very close to their stars.

The final method is the Transit Method. If a fly passes in front of a bright light, you can see a slight decrease in the light. If a bird passes in front of a light, you’ll see a larger decrease in the light. Same thing here: A planet passes in front of the star and temporarily blocks part of the light from the star that we would see at Earth. The big issue with this method is that you have to have the fortuitous geometry alignment where the planet’s orbit is just right so that it passes in front of its star as seen from Earth. The first one wasn’t detected until 1999, but a decade later, the dedicated spacecraft COROT and then Kepler were launched to look for these, monitoring the same fields of the sky, tens of thousands of stars, moment after moment, looking for those brief transits. In 2014, Kepler released over 800 planets discovered with this method, more than doubling the total number known, and that was on top of its other releases and, to-date, it’s found over 1000.

The transit method, despite the issue of geometry, is probably the best initial method. If you have the planet going in front of its star, then you know its alignment and you can follow-up with the radial velocity method and get the mass. Otherwise, the radial velocity method can only give you a minimum mass because you don’t know how the system is oriented, you only know that radial component of velocity, hence its name.

With the transit method, you can see how much light is blocked by the planet. Knowing the star’s type, you can get a pretty good estimate for the star’s size, and knowing how much light is blocked means you can get the cross-sectional area of the planet and hence its diameter. For example, Jupiter would block 1% of the sun’s light, and since area is the square of length, that means Jupiter is about 10% the sun’s diameter. Since the sun is a type G V star, we have a good model for its radius, though of course we know its radius very well because we’re in orbit of it. But that means not only can we get mass, but we can get size and density.

The transit method also lets us see if there’s a large atmosphere. If the light from the star instantly blinks down to the level when the planet passes in front of it, then any atmosphere really thin or nonexistent. If there’s a gradual decrease, then it’s extended. If its extended, we can follow-up with something like the Hubble Space Telescope and actually figure out what that atmosphere is made of by looking at what colors of light from the star are absorbed as it passes through the planet’s atmosphere.

And as with the radial velocity and timing methods, we know how long it takes to go around its parent star, and along with the star’s mass from what kind of star it is, we can get the distance of the planet from the star.

Okay, so much for a brief overview. But for me, I’ve left out a lot.

Moving on, it should be somewhat apparent that the bigger the planet, and the closer to its star, the easier it is to observe with pretty much ANY of these techniques, except direct imaging or microlensing where you want a big planet that’s far from its star. Big means big effect. Fast orbit means you don’t have to observe it for very long to show that it’s a regular, repeating signal best explained by a planet.

So, the question is then, can we detect an Earth-sized planet, and can we detect an Earth-like orbit? These are really two different questions and they depend on the technique you’re using. If we want to focus on a the two main methods – radial velocity and transit – then the unsatisfying answer to the second is that we do finally have good enough technology, it is just a matter of finding it. With the 2014 Kepler data release, there were over 100 exoplanets that are less than 1.25 Earth’s size. With the 2015 release, there are a total of 5 planets smaller than Earth or Venus, but they orbit their 11.2-billion-year-old star in just 3.6 to 9.7 days.

Even if we have observations for more than a year or two, for something as small as Earth, the level of signal relative to noise in the experiment is still pretty small, and you want a big signal relative to the noise. It’s best to build up multiple years’ worth of data to average out the noise to be able to really say that we have an Earth-like planet. For something like Jupiter, which orbits our sun in about 12 years, we’d need to observe at least two transits, meaning we’re just now approaching the time when we would have a long enough baseline of data with some ground-based surveys, but that’s also assuming we catch that planet for the few hours or days when it goes in front of its star versus the years and years that it doesn’t, and that we do this repeatedly and don’t chalk it up to sunspots.

This is why we really need long-term, dedicated surveys to just stare at the same place in space, constantly, measuring the light output of these stars to see if we can detect any sort of dimming, that’s repeated, from a likely planet.

But, even if we find an Earth-like planet in terms of mass and diameter and location in its solar system, that’s not enough to say it’s Earth-like in terms of atmosphere and surface gravity and overall long-term habitability. It’s just a first step. A first step we have yet to truly reach, but one that is reasonably within our grasp at this point.

But it’s from the existing planets we know of that we get some of the hype that hits the headlines every few months, like “astronomers estimate billions of Earth-like planets exist in our galaxy alone.” I’m not going to say that’s fantasy, but it’s loosely informed speculation based on extrapolating from a few thousand examples we now have from a very, VERY young field of astronomy.

Or, we’ll get articles where the first sentence says, “Astronomers have discovered two new alien worlds a bit larger than Earth circling a nearby star.” It’s in the next paragraph that we learn that “a bit larger than Earth” means 6.4 and 7.9 times our mass, and they orbit their star in just a few days.

So as always, this is a case where, when we see headlines, we need to be skeptical, NOT susceptible to the hype, and read deeper. But that said, it is entirely possible that any day now we will find an exoplanet that is at least like Earth in mass, size, and distance from its host star.

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Podcast Episode 61: Special Pleading with Large Impacts

A complaint I’ve heard is that the invoking of giant asteroid impacts to explain some odd solar system features (Venus upside-down, Uranus on its side, etc.) is just special pleading and as crazy if not more so than the pseudoscience ideas, like Velikovsky. While I obviously have my own opinion about Velikovsky in particular, I wanted to take an episode to talk about why giant impacts are used to explain some things, and whether we have a real reason to do so or if it’s just our own way of making stuff up.

There isn’t a new puzzler, though the one from last episode – send in your favorite planetary pareidolia – is still going on.

Podcast Episode 42: Who’s Yo Mamma?! (Milky Way or Sagittarius Dwarf?)

Episode 42 has been posted — on time, I might add. We’re back to the 30-minute episode length and get back to some good ol’ Coast to Coast AM clips.

I take you on a whirlwind custody case that’s 10 years (or 5 billion years?) in the making, trying to figure out if our solar system is really a member of the Milky Way galaxy, or is it a member of the Sagittarius Dwarf galaxy, a galaxy that was only discovered in 1994 and is currently being eaten alive by the Milky Way.

I also have another crater-based Q&A, discuss the solution to episode 40’s puzzler including the feedback that everyone sent in on what the fate of the puzzler should be, and then a few quick announcements.

Proof in Science versus the Media, Comets and Water, and Creationism

Introduction

How Earth got its water is an ongoing question in solar system evolution. A new study suggests comets are more likely to be the answer than previously thought. But could the answer simply be too hard for us to figure out; should we just not worry, and can we simply say that a loving God did it?

The Problem

When the solar system formed, there was a basic temperature gradient — it was hot in the center, where the sun was forming, and it got cooler as you went farther away from the nascent star. The location in the solar nebula that was about 100°C (212°F), is called the “Frost Line” where the water molecule would no longer be a volatile gas, but it would be a liquid and could be accreted to a forming object in an appreciable quantity. The frost line is about where the asteroid belt is.

Hence the problem: If liquid water could not form where Earth was, then how did Earth become the relatively water-rich world it is today?

A Solution?

For the last few decades, the favored solution has been delivery by comets. Comets are mostly water-ice, we know they impact objects, and we know that the impact rate was much higher in the very early solar system than it is today (in fact, I’m attending a workshop on the early solar system bombardment history in February where the focus will be on this).

A problem with this has to do with what’s called the deuterium/hydrogen ratio. Basically, water comes in two forms, “normal” water which is the familiar H₂O (two hydrogen atoms and an oxygen atom), and HDO (one hydrogen, one deuterium, and one oxygen). The latter is known as “heavy water” and you may have heard about it in relation to nuclear fusion.

Deuterium is a heavy form of hydrogen. A normal hydrogen atom has one proton in the nucleus. Deuterium has one proton plus a neutron, making its mass about twice that of a normal hydrogen atom … hence “heavy water” when it’s incorporated into the water molecule. It’s still considered hydrogen because the number of protons is what determines what atom it is. (And for those who like the extra credit information, tritium would be one proton and two neutrons.)

Getting back to the problem, the deuterium/hydrogen ratio (abbr. as D/H) is the normal ratio of heavy water molecules to normal water molecules found on an object. Earth’s oceans have a D/H of about 1.56 x 10^-4, or basically a bit more than 1 out of every 10,000 water molecules is heavy water. Comets, though, have been measured to be about (2.96±0.25)x10^-4, or around 70% too high. Asteroids are too low at (1.4±0.1)x10^-4.

So where did the water come from?

New Proof that Comets Watered the Earth

So proclaimed the title of an October 11, 2011 Time article. That’s right, “Proof.”

My problem with this statement is that we never have absolute “proof” in science. We have evidence that adds to the “conclusivity” (yes, I just made up a word) of a hypothesis. Proofs are in mathematics. Proofs never apply to real life. If you’re interested in this subject, I’ve written probably two relevant posts on it (post 1, post 2).

The article in question (Hartogh et al. 2011) is about a recent Nature Letter (a very short paper) that measured the D/H value in a comet named 103P/Hartley 2. The D/H measured in that comet came out to be (1.61±0.24)x10^-4 … which overlaps with Earth. This particular comet was from a different part of the solar system than previous comets with a D/H measurement, which is part of why this is a new result and why it was hyped up a bit.

The effect of this work is to revitalize the comets delivering water hypothesis, clearing up one of the biggest problems with it: We now do have a potential source for water that matches a significant constraint.

If you’re interested in reading more about it, other than the title, I do suggest the Time article.

But I Thought Goddidit

This brings us to the Answers in Genesis’ “News to Note” from October 15, 2011, specifically the second item. They don’t necessarily dispute the basic science of the article, rather the “view:”

“Nevertheless, in an effort to avoid a biblical explanation for the origin of all things—in other words, God as Creator—many cling to this explanation despite its aberrant physics. While the isotope ratios in the comets and asteroids are of scientific interest, they tell us nothing about the origin of the solar system. …

“The Bible explains the origin of the water on Earth and the origin of the entire universe. And the time of this Creation, about six thousand years ago, does not exceed maximum comet lifespans or demand a hypothetical birthplace to replenish them. … [God] made the Earth with its generous supply of water, not as a hot molten world that would boil away its water. After providing the Earth with an atmosphere, dry land, and plant life, He created the solar system and the other stars. He specifies that He made the sun, moon, and stars on the fourth day of Creation week. There is no way to blend the Genesis account of Creation with secular ideas of cosmology such as the big bang and the nebular hypothesis without calling God a liar.”

I really don’t think at this point that I need to go into detail about this and my position on it. It really is interesting to see, though, how these people are so willing to stick their heads in the sand and would be perfectly content in the Dark Ages of Europe a thousand years ago.

Final Thoughts

This was an interesting piece of science news, one that I knew some creationist somewhere was going to have an issue with, and one that I hoped the news media would not spin too broadly. I was right on the first, wrong on the second. With the latter point, these things are subtle, but using words like “proof” or “prove,” “hypothesis” versus “theory,” and “believe” versus “think” are words that shape significantly the public perception of science, how it works, and how “definitive” it is.

After over half a decade of fighting, the science-/evidence-based medicine crowd has succeeded in making that the term people use for what had been generally referred to “western” medicine. It’s a long battle, but maybe some day we’ll be able to get people to use some of these basic science terms correctly. At least when referring to science.

Solar System Characteristics that Do Not Point Towards Creationism (Though Claimed To)

This post is in regards to the Institute for Creation Research’s January 13, 2001 program entitled, “Sun, Moon, & Stars.” You can listen to the audio here.

The basic premise behind this episode of ICR radio is to talk about the sun, moon, and stars, and to raise enough questions as to lead a listener to think that these “three” celestial items prove the Universe was created.

The first train of thought has to do with one of the first real scientific ideas of how the solar system formed:  The “Nebular Collapse” theory that was thought of by a fairly famous mathematician named Laplace.  The Nebular Collapse theory’s basic premise is that a large cloud of dust and gas would collapse, the main center of collapse being where the star would form, and other points that started out as higher density eventually forming planets.  The ICR episode goes on to say that this is still the basis for how we think solar systems form today, which is true – it is the basis for it.

However, the program then goes on to discuss the debate that came to a head in the 1920:  Were “spiral nebulae” actually nebulae in our own galaxy that were in the process of collapsing into solar systems, or were they actually outside of our galaxy, being their own “island universes.”  The answer turned out to be the latter, when Edwin Hubble (you may recognize that name, there’s a fairly famous space telescope named after him) discovered individual stars in the “Andromeda Nebula,” thus proving that it was not a collapsing solar system.

As far as I can tell, the only purpose of bringing this up is an ad hominem attack on Laplace’s Nebular Collapse theory:  Because Laplace was wrong about these galaxies being nebulae, his whole theory of solar system formation must be wrong, which means that since it forms the basis of our current theories, they must also be wrong and so we have no idea how solar systems form.

If you listen just a little further (starting at 3 min 50 sec), you’ll find that my supposition (which I made before listening to the rest of the episode) is correct:  They use it to cast doubt upon our current theories.

They have Tom Henderson who used to work at NASA’s Johnson center (getting a former NASA employee to say this for them really helps their Argument from Authority fallacy) to talk about how the “Evolutionary Theory that the solar system formed by some solar nebula …”  can’t explain what we see today.  (Yet again, as seems to be the theme for these Creationism posts, I have NEVER introduced myself – nor thought of myself – as an “evolutionary” astronomer.)

For example, he points to Venus’ spin.  He correctly states that Venus revolves “backwards” on its axis, which “shouldn’t” happen according to the Nebular Collapse theory.  Well yes, that’s true, everything should be orbiting and spinning the same way.  But we can fairly easily explain it by a giant asteroid impact early on in the solar system’s history.  In addition, there are other possible mechanisms for flipping Venus over, none of which involve God.  The alternative that they imply but don’t explicitly state, of course, is that they want the listener to think that God must have created Venus just as it is.  But there’s no way to test that, no way to model it in a computer, and no way to make predictions based upon it.  In other words, it’s not science.

Next up, around 5 minutes into the program, they have Wayne Spencer talking about Saturn’s “Dancing Moons,” Janus and Epimetheus.  What’s intriguing about these two moons is that they are separated by only about 50 km from each other in their orbital distance from Saturn, which is smaller than many large cities.  And, every 4 years, they swap orbits.  Wikipedia actually has a decent section on this.  Wayne says the fact that they don’t collide must be evidence that God Did It.  However, computer modeling of the break-up of a large object and then what would happen to the fragments show that this kind of thing really can happen under the normal laws of physics and can remain fairly stable, again, not needing the Hand of God.

He then makes a side-note about how there are lots of surprises out there that we wouldn’t predict based upon a naturalistic worldview. This is correct. And that’s what makes science interesting: We’re always finding things that we can’t explain at that time, and then we work to try to understand why it is the way it is.

The broadcast then introduces Donald DeYoung (about 6.5 min. in) to discuss that ocean circulation is largely due to the moon creating tides, this being essential for the ocean’s health and that we require the ocean to be healthy for life, and without the circulation there would be no oxygen which he then equates with air for us to breathe. This goes from true to possibly to wrong. Yes, the oceans get stirred up quite a bit through the tidal effects the moon has on Earth. Maybe this is a requirement for life, but it’s unlikely because there are other mechanisms for circulating the water, such as winds as well as the simple rotation of the planet. There could still be plenty of biota in the oceans without the moon, it just may not flourish as much. And as to oxygen for us to breathe, this is not correct, for land-based plants also make free oxygen, not just ocean plants, and if there weren’t oxygen in the atmosphere for us to breathe, we may just as well have evolved to take advantage of some other gas that was plentiful in the atmosphere.

They then go on to discuss the moon’s stabilizing effect on our axial tilt, also known as obliquity (presently at the oft-quoted number of 23.5°). This is true – Earth’s tilt changes only by about ±1° because we have the moon helping to stabilize us. Mars, without a large moon, wobbles chaotically ±5-10° on the “short” timescales of millions of years, and between 0° to nearly 90° on much longer timescales. This ensures that our seasons are fairly steady and we don’t have the kinds of temperature extremes over various sections of the planet that we may otherwise have. It is entirely possible that life at our complexity could not have evolved on a planet with the kind of obliquity that Mars has. But this does not mean the moon was created — in fact, one could just as legitimately ask why (a) g/God didn’t create us on a planet WITHOUT a large moon for stabilization, just to show that s/he could?

The broadcast then (about 8.5 min.) starts to talk about Earth being unique and designed for life. This is actually a fairly straight-forward logical fallacy, the Argument from Final Consequences. In other words, they argue God must have created Earth because Earth is suited for life. This is not how science operates: They should be saying, “Earth seems to be suited for life as we know it, let’s try to find out why.”

After this, the program reverts to standard creationist arguments that don’t really have to do with astronomy, so I will end this post here.