Saturday, August 20, 2016

The Gaia data release and Tabby's Star - the ELI5

On the 14th of September 2016 we are expecting the first data release from Gaia, and it could well
Exploded view of the Gaia Probe
turn out to reinforce, constrain, or rule out some favorite conjectures about the weird behavior of a star romantically named KIC 8462852, aka Tabby's Star - behavior that was discovered by exploring the data from the Kepler Space Telescope.

Now, this is going to get pretty elementary, so if you feel you're already up to speed on the topics in the last paragraph, you may want to skip this.

Why Tabby's Star is weird is covered in this earlier entry. The TL;DR on that is that this star was showing bizarre variations in brightness completely inconsistent with the kind of star it is. We think that we know how the brightness varies, but can't be exactly sure how bright the star is in absolute sense - how many

The NASA Kepler probe that performed these observations is tightly optimized to do just that - measure variations in brightness of stars. However, this means that it's not particularly well suited to making super precise measurements of star positions. Fortunately, the European Space Agency has a sophisticated satellite in space right now that is exactly that - it can measure the distance to over a billion stars using something called the parallax method that I will discuss in a moment. It can also measure the color of stars and how fast they are moving away from or towards us.

The Parallax Method

The nice thing about parallax is that it is really simple, and you don't need to make any assumptions about a star to measure its parallax. Essentially, we watch a star for a year or longer, and make note of the very tiny variations in its apparent position as the Earth moves around the sun. Simple yes, but since the diameter of the Earth's orbit around the sun is only roughly 0.00004 light years, and the nearest star is just over 4 light years away, it takes a very clean, precise measurement to get this right.

If you want an obvious example of how this works, stand near a bookshelf loaded with books and hold your index finger straight upright in front of your face. Now, close your right eye - the finger will block your view of a particular book on the shelf.  Next, open your right eye and close your left eye. The finger now blocks a different book on the shelf. Finally, step back about twice as far from the shelf and try this again. You will see that the two different books blocked by your finger are now closer together. This is how parallax varies with distance - your two eyes are the same distance apart,  but the distance to the bookshelf is the interesting variable. An aside note is the amazing job your brain does integrating information from two eyes at the same time - play around with it.

Let's take a simple look at the math. It turns out that the amount the position in the sky varies over a year - the parallax -  is inversely proportional to the distance -  if star A is twice as far away as star B, the parallax of A is half that of B, since the EArth's orbit is reasonably circular. So, using Wolfram Alpha, we'll figure out about what it is for a star 4 light years away, given that a light year is about 9.5 trillions kilometers.

That number is an angle in units of degrees (the 180/pi factor at the end converts it from radians). That is a little sliver of a degree, or slightly more than one arc second, or 1/3600th of a degree. Maybe you can imagine just how hard it is to tell if a star is moving against the background of more distant stars and galaxies by that very slight angle. 

When I was a college student, all the parallax measurements were done with ground based telescopes, and only a handful of stars had accurately measured distance. The problem is that the Earth's atmosphere smears out the light from a star enough that if it is not one of the closest stars, it's parallax becomes impossible to determine.

Then, in the late 1980s, came the Hipparcos mission. Because Hipparcos used a space based telescope, it was able to measure the distances to many more stars using the parallax method - more than 100,000. This was a remarkable achievement and allowed us to recalibrate the distance scales of the entire universe.

Now, more than 20 years later, we have European Space Agency's Gaia probe. Bringing much more advanced technology to the job of measuring distances, Gaia promises to provide accurate distance measurements for more than 1 billion stars. Even Tabby's star, which is probably hundreds of times further away than the nearest stars, should have its parallax accurately determined for the first time. It was, unfortunately, just a bit too dim and distant for Hipparcos.

Parallax measurements only depend on the distance to the star. It doesn't matter what type of star it is, or anything else. So, when we get the parallax measurement to Tabby's Star, we'll be able to compare
that to the distance measurement made by measuring its brightness, based upon how bright we think it should be based upon what kind of star it appears to be from its color and so forth. For Tabby's Star, Gaia should be able to measure  the parallax down to about 25 millionths of an arcsecond, which is really good, and should give us an accurate parallax.

Why is this interesting? It is because a couple of different scientific investigations have reported results that show the star unexpectedly dimming on different time scales - a century, and a few hundred days. Montet and Simon's paper provides a fine description of this if you want the details. I interviewed Ben Montet on the Wow! Signal, and got him to explain even further.  Think about it - if this is true, it begs the questions of how bright the star was to begin with before it started dimming. Is it possibly considerably closer (weird, but supportive of the dimming hypothesis), or farther away (even weirder), than the current estimate says?

we'll know on September 14th. Whatever the result, it will be of great interest. However, don't get your alien magastructures out just yet - we also don't understand how ET technology could have caused the date we observe. We simply haven't been creative enough, and that would also apply to natural explanations as well.

Saturday, August 6, 2016

Aliens, Perhaps, but Not the Aliens of the Gaps

Update (8 August 2016): Audio Interview with Ben Montet.

With the publication of Montet and Simon's arresting new preprint showing even more anomalous behavior by Tabby's Star,  a lot of reasonable people are asking whether it's time to declare this stellar weirdness the work of an ET civilization, or whether it may be soon. While I am emotionally inclined to go this way, and intuitively sense that this maybe the ultimate conclusion reached, I am not a believer. There is a fundamental error we still must avoid.

Light curve for KIC 8462852 from Montet and Simon
It is not crazy or deluded to think that this could be the work of ET. Not at all. We know that technological civilizations exist in our galaxy, we just don't know how many. It is easy to get into pointless arguments about whether there is just one or the universe is swarming with creatures in some ways analogous to dexterous, talking monkeys like ourselves. These arguments are usually based upon probability guesses with very weak, or even non existent empirical support.

The truth is that nobody really knows how common ET civilizations are, or how long they flourish, and the so far null result of our (so far) very poorly funded SETI enterprise isn't much help in resolving it one way or the other, as has been argued by such persons as Jill Tarter for many years now.

So, saying that it could be ET is not the big mistake. The real mistake is closely analogous to an old argument for the existence of God, now largely abandoned by educated theists - called The God of the Gaps argument. In this argument, what we don't know about our origins or how the universe works is attributable to divine intervention - miraculous actions He must take to bring about that which nature can not - as if the nature of His design is somehow deficient. As science closes down the gaps, this god becomes smaller and smaller, and I think you can see why this would be unacceptable to enlightened religious people.

If we can't argue for the existence of God from the gaps in our understanding of human origins, neither can we argue for aliens based upon the gaps in our understanding of astrophysical phenomena. It's really just re-labelling our ignorance as "aliens." No one should be convinced by this.

Does this mean there is no scientific path to detecting ET? Of course not, and the decades of slow and patient SETI research point the way. Simple, but testable models of how an ET civilization would choose to advertise its presence are put forth, and there have been quite a few of these. These make testable predictions of what these beacons would be like and how they can be searched for.

Likewise, much of the focus of research in astrobiology (also with a null result to date) is focused on the question of how will we know extraterrestrial life when we see it? What is clearly different about the observables of a planet (either through a telescope or up close) between one that has life and one that doesn't? This leads to guidance for instrumental, experimental and observational design, going back to Viking lander of the 1970s.

For phenomena like Tabby's Star, we have to do the same hard work, but it is far more complex than SETI. in SETI, we generally assume that ET wants to reach out to other technological life forms, and will pick a method for communicating with us that is least in principle accessible to us. In other words, in SETI we can assume we know what ET is up to  - or were up to, as they may be long gone. For the sort of highly energetic side-effects of ET activity such as a Dyson Swarm, we probably don't know what they are up to, or why.  We may not even have a concept to express it. We will have to guess. These guesses would help us determine where to point our telescopes, what parts of the electromagnetic spectrum to search in, and what specific signatures to search for.

And we will have to be patient, since we will be almost certainly be wrong at first, or perhaps just unlucky in our search. We don't need to nail it exactly, but we will need to develop rough models of ET activity that distinguishes it from nature. These models would more or less fit the data that we think anomalous, would make testable predictions, and would show how to rule out at least known natural phenomena. Such a family of models may be available next year, or it may be in 100 years, but the more anomalous data we have, the more the models can be constrained.  The existence of serendipitously discovered phenomena like Tabby's Star certainly motivates this kind of work, and may even stimulate funding to undertake the search.

When this work is done, and the data is in, maybe we will know with reasonable confidence that ET Civilizations exist not far from us. It is impossible for us here and now in 2016 to see beyond that point. The thought of our children standing on that threshold and looking out into a living universe with newly enlightened eyes is thrilling to me, and I hope I live to see it.

update: David Grinspoon just published something similar.

Thursday, August 4, 2016

Ok, it just got weirder

A huge development tonight in Tabby's Star with publication of Ben Montet's preprint. More soon.

There will be a hangout tonight to talk about it. Message  me if you are interested in coming into the hangout, but we are limited to 10 people total.

Sunday, July 24, 2016

Messing Around in Aladin

Last Update: 16 August 2016

Looking into some of the stories I've been covering lately that live right in the imprecise border between astronomy and SETI, I've gotten interested in astronomical catalogs. It turns out that there are a lot of them, compiled over the years by a number of different scientific groups for different purposes. With the advent of astronomy outside the visible spectrum, the number of catalogs has multiplied, and it can be a daunting job to sift through them. I invite you to join me in my confusion and delight as I attempt to navigate my way through this glorious mess our civilization has built.

Aladin Sky Atlas is astronomy software made available for free from the University of Strasbourg in France. It gives you a graphical interface to a wide range of astronomical catalogs and image libraries. One thing it lets you do is overlay various catalogs across the electromagnetic spectrum around an object, so you can see for yourself what's nearby an object of interest and what its known properties are.

It turns out astronomers have cataloged far more objects than they have been able to study closely. As a result, there are many things not known about most of the cataloged objects. These are nearly all things that could be known if someone had the time and resources to look into them, but no one has yet.

Wednesday, May 18, 2016

Help catch Tabby's Star in the act - new kickstarter

Tabetha Boyajian and team have posted a kickstarter to buy telescope time to monitor KIC 8462852 photometrically around the world, 24 x7 using a network of telescopes. When a definite dip in brightness is detected, then hopefully the astronomical world will respond by swinging their more sensitive spectrographs and other detectors onto the star, allowing us some hope of really understanding what is going on around this very weird star.

This would compliment, not replace, the work that the AAVSO volunteers are doing.

I would hope you can see your way clear to donating to both efforts. If enough people show that this matters, it will happen, and maybe, just maybe, a new door will open and we'll see for the first time what is on the other side.