Thousands of exoplanets!

I was quoted in the newspaper today. One problem with talking to journalists, is that you don’t always know quite how they’re going to represent what you said, or – even – if you’re going to end up having said something silly; you don’t get much warning and you, typically, don’t get a chance to proof read what they end up writing. This article, however, seems fine; I’m not sure if I actually said what I’m quoted as saying, but it seems pretty close to something I would have said.

The article itself is about the recent announcement, by NASA, of 1284 new exoplanets. Just in case anyone doesn’t know, an exoplanet is a planet in orbit around a star other than the Sun. These new exoplanets were discovered by NASA’s Kepler satellite, which uses the transit method. The transit method basically works by staring at as many stars as possible (150000 in the case of the Kepler satellite) and trying to find those that show periodic dips in brightness. This would indicate something passing in front of the star. The relative dip in brightness can then be used to infer the radius of this object, and the period can be used to infer its distance from the star.

One problem with this method is that there can be lots of false positives; there are many things that aren’t planets that can cause what appear to be periodic dips in a star’s brightness. However, the Kepler data is so exquisite that they can rule out many of these false positives. That’s what’s happened here. These new exoplanets were amongst many candidate exoplanets detected a few years ago. The analysis now indicates that these 1284 candidates are almost certainly exoplanets, and hence have been announced as such.

This gives me an opportunity to discuss some of my own research. As the article says, I’m part of the HARPS-N consortium. Although the transit method has been extremely successful, it essentially only allows one to determine the radius of the planet and its distance from the star. If there are multiple planets, one can sometimes infer the planet masses from the timing of the transits, but this doesn’t work for all systems. However, in a planetary system, the star and planets all orbit the common centre of mass. This means that at some times the star will be coming towards us, and at other times away.

Telescopio Nazionale Galileo

Telescopio Nazionale Galileo

HARPS-N is a high-resolution spectrograph, part built in Edinburgh and located on the 3.6m Telescopio Nazionale Galileo. What it does is measure small shifts in the star’s spectrum which can then be used – via the Doppler effect – to determine the star’s radial (line-of-sight) velocity:

\dfrac{\Delta \lambda}{\lambda} = \dfrac{v_r}{c},

where \lambda is the rest-frame wavelength of a specific spectral line, \Delta \lambda is the shift in this wavelength, v_r is the radial velocity of the star, and c is the speed of light. From these small shifts in the spectral lines, you can determine the radial velocity of the star. If the radial velocity of the star shows periodic features, then one can infer that it must have companions (planets) and one can use this to infer the mass of these companions, their distance from their host star, and the eccentricity (or the circularity) of their orbits.

Credit: Motalebi et al. (2015)

Credit: Motalebi et al. (2015)

The figure on the left shows the radial velocity curves for 3 rocky planets in a 4 planet system that we discovered last year. I should be clear that the radial velocity is that of the star, and each radial velocity curve has removed the contribution due to all the other planets in the system. The top two curves are quite sinusoidal and indicate that these two planets are on roughly circular orbits. The asymmetry in the bottom curve indicates that this planet’s orbit is somewhat eccentric.

Okay, this post is getting rather long, but we’re getting to the point I was wanting to highlight. If you look at the figure on the left, you’ll note that the radial velocity amplitudes are a few m/s. The spectral resolution of HARPS-N is R = 115000. This is the inverse of the smallest relative wavelength change that can be measured by the instrument

R = \dfrac{\lambda}{\Delta \lambda}.

If you look at the formula for the Doppler shift that I included above, you can relate this to the spectral resolution through

R = \dfrac{\lambda}{\Delta \lambda} = \dfrac{c}{v_r} \Rightarrow v_r = \dfrac{c}{R}.

If HARPS-N has R = 115000 and c = 3 \times 10^8 m s^{-1}, then v_r = c/R = 2609 m s^{-1}. Hmmmm, if this is the smallest radial velocity that we can measure, how can we have measured radial velocities of only a few m/s? The reason is that we measure across a wavelength range (383nm – 690nm) where there are lots and lots and lots of spectral lines, and then we cross-correlate with the known spectrum of the type of star we’re observing. The peak in the cross correlation function then gives the wavelength shift, from which we can determine the radial velocity of the star. You then need to repeat this a number of times (maybe 30 to 60) over the course of a year or so, to then produce the radial velocity curve from which you can determine if there is a companion planet, and – if there is – the properties of that planet.

So, even though we can’t directly determine the shift in individual lines, we can still determine the wavelength shift and – hence – the radial velocity of the host star. Given that we can’t actually see the shifts directly, how can we be confident that what we’ve measured really is indicative of a companion planet? One way is that different teams observe the same system and get the same result. Another is that some of the systems we observe are Kepler targets that are already known to probably host planets. The radial velocity results for those systems are consistent with what is already known from the transit measurements. Finally, and this applies to the 4-planet system I mentioned earlier; some of those detected via the radial velocity measurement are then found to also transit their host stars. Again, the results are consistent.

Mass-radius relations for exoplanets (Motalebi et al. 2015)

Mass-radius relations for exoplanets (Motalebi et al. 2015)

Maybe I’ll finish by pointing out another reason why combining radial velocity and transit measurements can be so powerful. The radial velocity measurements give the mass of the planet, while the transit meaurement gives its radius. Together they give the density, from which one can infer the internal compostion. The figure on the right shows the mass-radius relation for a number of known exoplanets, including Kepler-78b (K78b) which our team characterised a few years ago and is still the most similar – in composition and size – to the Earth, and HD219134b, one of those shown in the radial velocity figure above.

What’s clear is that there are a number of known exoplanets with compositions that appear very similar to the Earth. However, to date, these are all planets that are very close to their parent stars and, therefore, are almost certainly far too hot to host life. To date, we do not know of any genuine Earth-like exoplanets, in terms of composition, size, and distance from a star similar to the Sun. This is one reason why I think we have to be careful when talking to journalists about this topic. It’s easy to make them think that we’ve found something Earth-like and, hence, habitable, when really it is simply a rocky planet with a composition similar to that of the Earth, but almost certainly too hot to harbour life. For the moment, I would be very cautious about accepting any claims of having found a habitable, or even potentially habitable, planet. In 10-20 years time, though…….

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44 Responses to Thousands of exoplanets!

  1. Eli Rabett says:

    Looked for NO2 in the atmospheres?

  2. Eli,
    We can’t easily characterise the atmospheres of the rocky planets yet. It’s possible for some of the more massive, gaseous exoplanets, but not for the lower-mass rocky ones. What made you ask about NO2, though?

  3. Magma says:

    Very interesting re. both the research itself (the rate at which we’re discovering exoplanets) and the trick* used to pull out signals that would appear to be hopelessly below instrumental resolution.

    * In the well-known Mannian meaning of the word.

  4. Magma,

    the trick* used to pull out signals that would appear to be hopelessly below instrumental resolution.

    That was partly what motivated this post, but I thought I didn’t need to explicitly make the obvious link to climate science 🙂

  5. Eli Rabett says:

    NO2 is about the only stable molecule with a structured emission/absorption spectrum throughout the vis/uv

  6. Eli,
    Ahhh, I see. Most of the rocky planets we’ve characterised have orbits of a few days or less (Kepler-78b has an orbital period of 8.5 hours). I would expect most of these are too hot to have any kind of atmosphere.

  7. Willard says:

    Measuring stuff for habitability is one thing, but I’d rather measure for hospitality.

  8. Ron Graf says:

    Anders, this is very interesting. I have always loved astronomy and SETI, and now they seem to be actually converging. How long until we can confirm an exo-earth complete with oxygen in the atmosphere? I suppose with your current work we will better know where to immediately look when we get that instrument and method.

    Which do you see as the timeframe for human civilization, if successful in achievment?
    a) Colonizing Mars and/or Moon
    b) Colonizing artificial space structures
    c) Living on interstellar space ships

    I left off colonizing exoplanets to not even tee up the idea of humankind becoming alien invaders.

  9. Ron,

    How long until we can confirm an exo-earth complete with oxygen in the atmosphere?

    Hard to say. Maybe mid-2020s if we’re lucky. This might be partly because we won’t have the instruments and partly because we won’t have suitable targets till then.

    I suppose with your current work we will better know where to immediately look when we get that instrument and method.

    That’s the idea.

    Which do you see as the timeframe for human civilization, if successful in achievment?
    a) Colonizing Mars and/or Moon
    b) Colonizing artificial space structures
    c) Living on interstellar space ships

    I really don’t know.

  10. Szilard says:

    Extremely cool – bravo!

  11. Ken Fabian says:

    What proportion of planets within range will not pass between us and their star and thus remain undetectable? It’s a big step again to be able to detect, perhaps by atmospheric composition, the presence of life.

    Humans as the interstellar invaders would be a likely outcome if we don’t lift our game with respect to environmental responsibility and internal governance and somehow those worlds became accessible; certainly sacrificing the endemic ecosystems of any world without existing ‘sentient’ life for the sake of human convenience is taken as given in most related fiction.

    Ron, my own (perhaps pessimistic) view is that it requires a large, prosperous and forward looking global economy for future colonising of space to happen and if they do they will long remain outposts of that economy, rather than self sufficient and independent. ie that if we fail to adequately face and address issues like sustainability and climate stability here on Earth that economic base will erode and requisite technological progress will be harder or won’t happen. Other views include the inverse; that without the resources of space we will fail our nearer term challenges and a massive commitment now is needed as some kind of humanity saving exercise. I’m doubtful of that myself; ultimately our most significant challenge isn’t to endlessly increase resource availability but to sustain ourselves within the existing limits of the best planet around.

    I think it must be qualitatively different to historical precedents down here, where potable water falls from the sky, air is breathable, existing environment can provide necessary resources for small groups of earliest colonisers carrying all they need on their backs and in their heads. Later versions, colonising already occupied territories was done on the back of existing, economically viable working infrastructure (ships, ports) and rather than built at sufficient scale entirely from scratch; there would have been a minimum threshold of investment and effort required to succeed. For space I think we must assume a much higher threshold.

    Strong Earth based economic incentives are needed to drive it, such as the lure of mineral resources, for the necessary minimum pre-investments to achieve viability but those don’t appear to have been sufficiently attractive so far.

  12. Pat Cassen says:

    Congratulations on a very exciting project and your excellent explanation of the research!

  13. Ken,

    What proportion of planets within range will not pass between us and their star and thus remain undetectable?

    The transit probability is

    P = \dfrac{R_s}{a},

    where R_s is the star’s radius and a is the distance of the planet from the star. So, for a planet at 1 AU (Earth-Sun distance) it would be about 0.5%, whereas for a planet at 0.05 AU (‘hot’ Jupiter, ‘hot’ Neptune, ….) it would be about 10%. That’s why you need to stare at lots of stars to detect transit events.

    On the other hand, if you find via radial velocity a planet that is very close in, the chance of transit can be high. That’s what happened with the 4 planet system we detected. The inner planet was so close that the transit probability was quite high and so we went and looked with another telescope and it did indeed transit.

    It’s a big step again to be able to detect, perhaps by atmospheric composition, the presence of life.

    Absolutely. Simply detecting signatures of atmospheres on rocky planets is going to be very difficult. Once we’ve done that, it’s still going to be hard to determine if the planet could harbour life, or not.

  14. -1=e^iπ says:

    “I was quoted in the newspaper today.”

    So are you going to change the ‘about me’ section?

    “For the moment, I’ve decided to remain anonymous.”

  15. John says:

    Anders, sorry a bit off topic but I gotta ask:

    What’s your take on star KIC 8462852 and the “alien megastructure” hypothesis? Much ado about nothing, comets, or… ?

  16. John,
    I don’t have strong views about that, but I think the disruption of comets makes more sense than some kind of alien megastructure.

  17. JCH says:

    Pretty neat… though now I really can’t figure out why you would waste any of your time on the foolish of climate skepticism. Glad that you do.

  18. Joshua says:

    =>> “Nope”

    If you did, you would deprive certain “skeptics” of the passive-aggressive thrill they get from being sure to refer to you by your RL name every chance they get.

    And in so doing, deprive me of the amusement of watching them get a thrill from trying to annoy you.

  19. Dikran Marsupial says:

    Joshua, yes, pathetic, isn’t it? ;o)

  20. Andrew Dodds says:

    Ken –

    Yes, I’d agree that until a minimum basic infrastructure is in place, we won’t see a ‘spontaneous’ expansion into space.

    You could envisage a moon base in one of the polar craters (24 hour sun easily available along with water and ammonia ice, hopefully) that was capable of growing it’s own food, and making fuel, oxygen, aluminium and magnesium panels. Doesn’t have to be a high tech manufacturing operation; the point is that getting stuff from the moon to earth obit is much, much easier than getting it from the Earth’s surface to earth orbit. But getting this base in place needs a large and sustained governmental effort.

    Has to be said that when Europe tried to colonise the New World, there were a lot of failed attempts as well as the successful ones, and the death rate was high enough for a long time that populations only grew or survived through immigration.

    As for travel beyond the solar system.. probably not as humans. As robots, possibly.

  21. Joshua says:

    Dikran –

    Yes. So juvenile.

  22. JCH,

    though now I really can’t figure out why you would waste any of your time on the foolish of climate skepticism.

    I can’t either 🙂

    Joshua,
    Indeed. I was commenting on one of John Cook’s conversation articles recently using my Twitter account. A number of the usual suspects were highlighting that one was meant to use real names, despite them obviously knowing it and – in one case – actually using it in the comment that pointed this out.

  23. The Very Reverend Jebediah Hypotenuse says:

    Back in 2009, I had the pleasure of attending a talk, and then having dinner with, John Troeltzsch, Kepler Mission Program Manager with the spacecraft manufacturer (Ball Aerospace). The Kepler spacecraft had just been launched the previous month, (after two delays due to funding problems) and was not yet stationed in its ‘Earth-trailing’ solar orbit, much less taking science data.

    What impressed me most about the Kepler Mission was the astonishing engineering that was required for it to even have a chance of meeting its science objectives – Beginning with (at the time) the largest mirror in space, and the largest CCD camera in space…

    At Troeltzsch’s talk, he confirmed that the primary focus had been successfully optimized – by moving the primary mirror 40 micrometers towards the focal plane and tilting the primary mirror 0.0072 degrees.

    Now – 7 years later – over 2,000 exo-planets have been detected!

    Very interesting to learn of the cross-confirmation of radial-velocity and transit measurements, and in particular, of the cross correlation derivation of the velocities.

    Good stuff! Congratulations, ATTP!

  24. John Mashey says:

    Good wood work … and reminiscent of paleoclimate work, in the sense of not being able to do lab experiments, but having to tease out insights from observational data, perhaps by better instruments or algorithms, often having to use proxies of one sort or another.

  25. guthrie says:

    Rule number one of talking to journalists is have your own copy of what was said/ emailed etc, so that if things do go wrong or you get flack for some misquote, you can say “I didn’t say that”. Some are genuinely trying hard to meet impossible deadlines, others are just phoning it in, and some couldn’t care less or are just stupid.

    [Mod: the latter]

  26. The Very Reverend,
    Kepler has been remarkably successful. Pity that’s two of its reaction wheels failed so that it couldn’t stay on the same targey field for long enough to have a reasonable chance of finding something really Earth-like.

    John,
    That’s what’s always struck me and is probably one reason why I found so much of the criticism of climate science so strange given that the underlying methods seem quite similar to astronomy, which doesn’t face anything like the kind of backlash that climate science gets.

    Guthrie,
    I tend to try and be quite careful and have never had a problem. Of course, I’m unlikely to end up saying something that will end up being criticised on some science denial site 🙂

  27. guthrie says:

    As someone who has read a lot of SF and stuff, I just want them to get on with a long baseline interferometry set of satellites out in space. Then we’ll get some more definite answers.

  28. Tamino’s background in astronomy allowed him to use statistical methods that even today many pseudoskeptics (i.e., virtually every electrical engineer) believe impossible. Electrical engineers learn the Nyquist Theorem and take it as gospel that the sampling rate must be at least 2fmax, or twice the highest analog frequency component. I’ve had to refer many of them to Tamino’s posts on sampling:

    Astronomy is plagued by sampling problems. We just don’t get to observe when we would like to. Most targets can’t be seen during the day, or when the sun or moon is too close, or when the weather doesn’t cooperate. And for professionals rather than amateurs, competition for telescope time at major observatories is fierce (which is a distinct advantage for amateur astronomers).

    That’s why astronomers are used to irregular time sampling. It has some disadvantages, but it also has advantages such as the ability to search frequency space far far beyond the “Nyquist frequency” or, frankly, any frequency limit you care to define. In my opinion, in all but very specific circumstances the advantages of uneven time sampling outweight the disadvantages, by a lot. Uneven time sampling is sometimes thought to be a bane for data analysis, but it’s far more likely to be a boon.

    ATTP, Tamino, & Willie Soon. Oh well, 2 out of 3 ain’t bad 🙂

  29. wheelism says:

    Pass the Meatloaf – Soon’s an aerospace engineer.

  30. Anders,

    Last time you posted an astronomy article I was one who encouraged you to write more of them. I’m not disappointed by this one. Interesting, brilliant research; congratulations to you and your colleagues on your continued successes.

    The only downside … not much to argue about. 😀

  31. wheelism says:

    (Appears that I spoke too soon [Mt. Wilson Obs. since 1992]. Sincere apologies.)

  32. anoilman says:

    oneillsinwisconsin: I resent that. I’m an electrical engineer. I live and breath sampling. I prefer a healthy 20 to 1 for time domain analysis, although higher is much better. Higher sampling rates allow you to do more funky analysis, and or use more accurate filters.

    All EE’s prefer higher sampling rates, and its only the sad few newbs who even try to go as low as the Nyquist Frequency.

  33. John Mashey says:

    Wheelism
    1) Soon’s PhD was in Aerospace, under Joseph Kunc.

    2) see Willie Soon and Friends for the back history.
    From p.24 of APS2009:
    “Sallie L. Baliunas (1954, 4GHO) was a young astrophysicist at the Harvard-Smithsonian Center for Astrophysics (H-S CfA) who wrote a 1985 paper cited in the GMI book, and whose Preface thanked her for help. She and Jastrow also published a 1990 paper, so they had already started a long working relationship. He became Director of the Mount Wilson Observatory 1992-2003, where she had visited since 1977. At some point, perhaps as early as 1992, she became Deputy Director there, and they often wrote papers together.

  34. wheelism says:

    John, I was recalling the NY Times article pointing out that the “Harvard astrophysicist” was neither. Of course, neither astronomy nor astrophysics whisper promises of wealth and romance in my near future, so I don’t pay much attention.

  35. Andrew Dodds says:

    Sampling problems? When I were a lad studying Geology, we’d be lucky to get a 3 square meter outcrop to represent a cubic kilometer of rock. If we found a cliff we’d think it were Christmas, I tell you. And we could tell you half t’geological history of the planet from a grain of zircon in a metamorphic sandstone… Astronomers don’t know they’re born. As for those pampered EEs..

    Also, I want to see a planet that falls below the ‘100% Fe’ line on that graph. That would be.. interesting.

  36. The Very Reverend Jebediah Hypotenuse says:


    When I were a lad studying Geology, we’d be lucky to get a 3 square meter outcrop to represent a cubic kilometer of rock. If we found a cliff we’d think it were Christmas, I tell you.

    Luxury.

    You were lucky to ‘ave outcrops.

    Back when I studied physics, we did our kinematics labs at the bottom of a lake. In shoe-box.

  37. Dikran Marsupial says:

    Excellent post, fascinating.

    TVRJH :o)

  38. Magma says:

    For the moment, I would be very cautious about accepting any claims of having found a habitable, or even potentially habitable, planet. In 10-20 years time, though…….

    I was thinking about this. I’ve noticed how bright Mercury and (in particular) Venus can be in the evening or predawn sky, and watched both of their recent transits using solar-filter equipped binoculars. A trivial calculation shows that from Earth an exo-Earth would have a (reflected) brightness10-4 to 10-5 times that of its exo-Sun. Do we have the capability of resolving such a hypothetical planet ~1 AU away from its parent star in the local stellar neighborhood?

  39. Steven Mosher says:

    Pass the Meatloaf – Soon’s an aerospace engineer.

    sure dont eat too much

  40. wheelism says:

    Steven: Thank you for the huge grin I’m wearing.

    oneillsinwisconsin: So sorry to have dismissed your observation.

  41. dbostrom says:

    NO2 is about the only stable molecule with a structured emission/absorption spectrum throughout the vis/uv

    Also may hint as to whether VW have made it to a particular planet. A market for VW implies advanced alien life.

  42. Pingback: A new exoplanet! | …and Then There's Physics

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