Dense exoplanet gets the lead out and in

By Phil Plait | May 5, 2011 7:00 am

Sometimes it pays to look over some older data and re-examine it. An exoplanet called 55 Cancri e was thought to have an orbit that was just 2.8 days long when it was discovered. However, two researchers looked over the data and realized they got a better fit if the orbit were actually only 0.73654 days — just under 18 hours! This meant it orbited its star far closer than previously thought as well.

And while that may be somewhat interesting, it’s the implications for the planet itself that make this orbital revision so cool. Or actually, hot. And dense.

Right. As usual, there’s a story to tell here…

The planet was discovered using the Doppler method: as it orbits its star, the gravity of the planet tugs on the star, causing a very small shift in the spectrum of starlight. The problem is getting enough observations to nail down the planet’s period; you can’t observe when it’s up during the day, and that cuts into the ability to get a good sampling of measurements. The discovery data gave a good fit at 2.8 days, so that’s what astronomers assumed was the orbital period.

But there were gaps in the data, and that can mask the true orbital period. When the data were examined more carefully, the 18 hour period was seen. But was it real?

Well, it turns out that the shorter period possibly meant there was another way to see the planet: transits! Some exoplanets have their orbits aligned just right so that from Earth we see the planet planet passing directly between us and the star once per orbit. The closer in the planet orbits, the higher the chance of such a transit taking place. An 18 hour orbit is pretty close! So astronomers predicted when the transits would take place and, using Canada’s MOST telescope, took a look.

Boom! Right on time, the planet was detected as a slight dip in the starlight… a very slight dip; only 1/50th of a percent of the star’s light. That’s pretty feeble and hard to detect, but it was definitely there.

But that tiny drop in light from the star tells a big story. First of all, it tells us the planet’s size (the bigger the dip in light, the bigger the planet). 55 Cancri e, it turns out, is only a little bigger than Earth: about 1.63 times the Earth’s diameter. This makes it a superearth; bigger than us, but not nearly big enough to be a jovian, Jupiter-like planet. And since the Doppler method used to detect it in the first place gives the planet’s mass — 8.6 times that of the Earth — we can calculate the average density. It’s a whopping 11 times that of water, twice that of our home planet. That’s about the same density of lead.

That’s weird. Really weird; it’s the densest exoplanet found by a long shot.

And this planet is hot. Freaking hot. It only orbit 2.3 million kilometers (1.4 million miles) above its star’s surface, so the temperature on the dayside of the planet may reach nearly 2700°C, or 4900°F! If the planet really does have a lot of lead, that metal is sitting on the surface in pools of liquid. At that temperature and mass, it’s pretty unlikely the planet could hold on to an atmosphere for very long, so it’s most likely airless.

So 55 Cancri e is a forbidding place. You’d weigh three times what you do on Earth. The sun would beat down on you, appearing 65 times bigger in the sky than it does here on Earth, about the size of a big dinner plate held at arm’s length (for comparison, from Earth the Sun is small enough to be easily blocked by your thumb held out at arm’s length).

And, of course, you’d burst into flame instantly… except there’s no air, so in reality you’d just cook right there on the spot. Or smolder, more likely.

Interestingly, a trend is starting to emerge: the densest "superearth" exoplanets tend to be very close to their star, while less dense ones are farther out. This makes some sense, since for a close-in planet the atmosphere gets lost rapidly, leaving the denser material behind. Still, we need more samples in this survey!

For years, my whole life, really, I used to go outside and look at the stars, wondering which if any had planets orbiting them. 55 Cancri is a naked-eye star (barely), Sun-like and only 40 light years away. While we’ve only been detecting exoplanets for a few years, we’re starting to get decent statistics on them, seeing trends and getting tantalizing hints at what’s to come.

I know the ultimate goal is to find Earthlike planets, uncover more brethren to our homeworld. But along the way we’re finding planets that are surprising and weird, and sometimes I think that may actually be where all the fun is.


Related posts:

- Gallery of exoplanets: real pictures of alien worlds
- Kepler finds a mini solar system!
- Exoplanet found from another galaxy!
- Does Gliese 581g exist?

CATEGORIZED UNDER: Astronomy, Cool stuff

Comments (39)

Links to this Post

  1. A nearby star may have more planets than we do! « | April 6, 2012
  1. How the heck does one accumulate that much heavy metal in one place? A quick perusal of a list of the elements by density shows there isn’t a lot of stuff denser than lead out there:

    http://chemistry.about.com/od/elementfacts/a/elementdensity.htm

    Maybe it’s a ball of gold planted perilously, unobtainably close to a star by some galactic prankster, like the Jovian diamond core in Arthur C. Clarke’s writing.

  2. Sam H

    This is amazing – since planet e’s orbit is aligned to make transits, this could mean that the rest of the system is as well – opening the possibility of directly observing planet f via transit, the Saturn-mass habitable zone giant which could hold Pandora-class moons in it’s orbit!!

    And a planet as dense as lead – wow!! Just a thought: might there be elements not yet on our periodic table out there, some of which could be denser than lead? If so, these elements may or may not have accumulated on planets like this – but who knows. This planet probably has a global ocean of liquid metal that may be evaporating directly into space. But why not a tenuous atmosphere? If the planet’s gravity is strong enough (which it seems to be, due to the high mass and large size), might it be able to hold onto a few wisps of the escaping gas?

  3. Rebecca Harbison

    Alex @ 1

    Solids are pretty incompressible, but not perfectly so. For instance, the Earth’s density is 5.5 g/cm3. But Earth’s uncompressed density — the density you would get if you blew up the Earth and then took the density of the debris, is 4.4 g/cm3.

    So, 55 Cancri e has to have a lot of metallic elements, as those are a lot more dense than rock. But the fact it’s 8.6 times the mass of Earth will help boost the density — even if its composition was exactly like Earth’s, it would have a higher density.

    (Gases are worse, of course. Until we started looking, it was widely thought that the radius of Jupiter was about the largest radius you could make a planet/brown dwarf — piling on more matter would just compress it more than add to the radius. The ‘puffy’ planets — planets 2 or 3 times Jupiter’s radius — are really interesting for that reason.)

  4. Thankyou BA – great write up of an astoundingly strange and wonder-filled exoplanet – a leaden SuperMercury? The exposed core of gas giant that’s been blown away? Something else again? :-)

    So superluminously marvellous that we’re able to learn so much about such distant (formerly!) hard to spot planets of distant suns. :-)

    Click on my name for another article on this exoplanet via “OneMinute Astronomer” too btw. :-)

  5. rws3n

    Hi Phil,

    See also arXiv 1105.0415 (Demory et al.), submitted to A&A on May 2. Their warm Spitzer measurements give a radius of 2.13 plus 0.14 minus 0.13 R_E. That gives a density around 5 g/cm^3. They note the 3 sigma discrepancy between their radius determination and that of Winn et al.

    Still, pretty cool considering when we started grad school the total number of known extrasolar planets was zero.

    Rob.

  6. Astroz

    Was there just measurement for 1 transit? Is it possible that it’s a partial eclipse? This can lead to underestimating the planet radius and thus its density.

  7. I wonder if the surface of 55 Cancris e* looks like this artistic depiction of kepler-10b? :

    http://www.youtube.com/watch?v=AqqIpdw4YeA

    Towards the 2 minute mark of that Youtube clip.

    Or maybe the surface looks like that in this familiar scene :

    http://www.youtube.com/watch?v=pSwy412nttI

    If the lava is particularly heavy & valuable – after all they were mining it there so it must have been pretty concentrated in riches. ;-)

    Oh & here’s the :

    http://en.wikipedia.org/wiki/55_Cancri_e

    wiki-page for 55 Cancris e which is apparently in need of updating! (At this time anyhow.) ;-)

    * Yegods it needs a better name methinks – may I be the first to suggest “Mustafar”? ;-)

  8. Dave

    Nice post Phil!

    There’s an addendum to the story already. Using the Spitzer Space Telescope, another group found the transit, but found a somewhat larger transit depth of 2.13 Earth radii. If the new volume is correct, the planet’s density is close to that of Earth. The truth might lie somewhere in between the two measurements, and the error bars might be somewhat underestimated for one or both detections. Other, more exotic, possibilities are mentioned in the new paper (link below), such as an evaporating atmosphere that is optically thicker in the infrared (where Spitzer observed) than in the optical (MOST). But for now, I think the prosaic hypothesis of observations that are consistent with each other, just with underestimated uncertainties, is perfectly reasonable. As usual, more data will pin this down. This type of observation can (as of now) basically only be done from space.

    Hubble might be able to resolve the matter, although maybe not since the star might be uncomfortably bright for HST. Phil, you could probably comment on this.

    http://arxiv.org/abs/1105.0415

  9. ceramicfundamentalist

    question: if the doppler method relies on collecting concrete evidence of the periodic movement of a star to infer the existence of a planet, what are the chances that an incorrect measurement of that movement would still turn out to be a planet in the end? wouldn’t it have been more likely that the “planet” was actually noise or some other error? this makes me suspicious of the doppler method.

  10. Douglas Troy

    Okay, so the scariest environment imaginable. Thanks. That’s all you gotta say, scariest environment imaginable.
    ;)

  11. toasterhead

    2. Sam H Says:
    May 5th, 2011 at 7:41 am
    This is amazing – since planet e’s orbit is aligned to make transits, this could mean that the rest of the system is as well
    —–
    Not necessarily. I think the key here is that this particular planet is so ridiculously close to its star that a transit can be observed even if the system isn’t perfectly edge-on to us.

  12. kevin

    2700°C, wow. I had to look up to be sure, but that’s not just above the melting point of lead. It’s above the boiling (not melting!) point of iron. So yeah, hot.

    But it’s reassuring to know that Carbon is among the 5 or so elements listed on wikipedia that won’t melt at that temperature, so the option of a visit isn’t completely ruled out, I guess. [http://en.wikipedia.org/wiki/List_of_elements_by_melting_point]

  13. Nobody Important

    55 Cancri (although I prefer the less-accurate Rho Cancri) is my very favourite star-system. It’s just so…familiar, somehow.

    But I wonder if 55 Cancri e is what’s called a “Cthonic planet”, a kind of planet that is just the leftover core of a gas giant. I mean, 55 Cancri is an old star – older than our own, in fact. Could it not be the case that a Saturn-sized, or otherwise small enough gas giant orbiting for long enough would lose its atmosphere, leaving behind just the rocky/metallic core?

  14. Mike Richards

    The materials this planet is made from need not be very exotic to produce such a huge density. The Earth’s Inner Core is essentially iron/nickel alloy which on the surface have densities of between 7 and 8 g/cm3. But seismic evidence shows that the Core has a density of between 12 and 13 g/cm3. No change in composition, just lots and lots of compression.

    55-Cancri is larger and more massive than the Earth, it will have squeezed its core even harder, which would have upped its bulk density.

  15. But Earth’s uncompressed density — the density you would get if you blew up the Earth and then took the density of the debris.

    That would make an awesome Beakman’s World segment.

  16. chris j.

    what grabbed my attention most about this post was the picture of what our sun would look like if an alien observer was looking for transits!

    Nobody @12:

    i had a similar thought. over an assumed lifespan of about 5 billion years, at those temperatures all of the lighter elements could have volatized and escaped into space – a process that could still be ongoing, giving rise to a transient atmosphere (as Dave mentioned).

  17. Sion

    As all of these things seem to be in weird and unexpected orbits, just a quick thought – we’re not missing some step in the maths are we? It’d be embarrassing to find that someone forgot to carry the one in an equation somewhere and actually most exoplanets are perfectly normal. Just askin’. ;)

  18. MaDeR

    My guess this is exactly is: remnant of gas planet, dense core of former gas giant that was completely eroded away during bilion years around star. We already discovered hot jupiters where this process is ongoing.

  19. rfguy

    And this planet is hot. Freaking hot. It only orbit 2.3 million kilometers (1.4 million miles) above its star’s surface, so the temperature on the dayside of the planet may reach nearly 2700°C, or 4900°F! If the planet really does have a lot of lead, that metal is sitting on the surface in pools of liquid. At that temperature and mass, it’s pretty unlikely the planet could hold on to an atmosphere for very long, so it’s most likely airless.

    But the boiling point of lead is just 2022K (or 2013K, depending on who you believe on the internet, but it doesn’t affect my point), so if the surface of the planet is about 3000K, then any lead at the surface would boil off. Perhaps it would have a lead-vapour atmosphere? Perhaps a mix of more common elements – iron, nickel, copper vapours? Has any modelling been done to see if a hot, dense planet could hold onto an atmosphere like this?

    -mark.

  20. Gonçalo Aguiar

    As dense as lead they say? Isn’t there the possibility that the data iswrong, or wrongly interpreted?

  21. ethanol

    I went out last night and looked at 55 cancri with my telescope. My 6″ is quite incapable of resolving the double star (or the two were in line), but it’s still cool to look at it and know that this planet is there, and maybe even passing in front of the star. As for the anomalous density, I think Rebecca is right. I can imagine that if a planet was very metal-rich like mercury and considerably larger than the earth, that the compression at its core could get pretty intense. It doesn’t actually have to be made of lead to have that sort of density.

  22. andy

    Given the two very different radii the question is now how to resolve the data. It might be instrumental artifacts (for example, Spitzer is not an actively-cooled space telescope any more). Alternatively the planet may have an extended atmosphere which strongly absorbs in the infrared but not at visible wavelengths – carbon monoxide or carbon dioxide could do this.

    Have to say I’m a bit sceptical of the Spitzer radius: the implied volatile content seems quite high for such a low-mass planet subject to such intense heating by the star. The more refractory the better!

  23. amphiox

    (Gases are worse, of course. Until we started looking, it was widely thought that the radius of Jupiter was about the largest radius you could make a planet/brown dwarf — piling on more matter would just compress it more than add to the radius. The ‘puffy’ planets — planets 2 or 3 times Jupiter’s radius — are really interesting for that reason.)

    A bit of an aside here, but this means that a super-jovian gas giant planet that isn’t puffed out by some other force with say 8-12J mass and the 1J radius would be substantially denser than lead. So the densest gas giant planets in the universe are going to be denser than the terrestrials!

    And taking this consideration even further, the very largest brown dwarfs, at 79-80J in mass, again in a 1J radius, and the very smallest red dwarf stars, at 80-81J mass in 1J radius (radiant pressure from hydrogen fusion would puff out stars again beyond this mass range, but at the very borderline this effect would presumably be small and the object still remain close to 1J in radius), are going to have densities around 100, about 10X that of lead! Which should make them the densest normal (not degenerate like the superdense stuff of white dwarfs, neutron stars and black holes) matter objects in the universe….

  24. Simon Green

    @Dave:

    “…Hubble might be able to RESOLVE the matter…”

    Ah, I see what you did there!

  25. andy

    This is amazing – since planet e’s orbit is aligned to make transits, this could mean that the rest of the system is as well

    As far as I am aware, transits of the next two planets out from the star have already been ruled out by observations made several years ago. The second planet (designated b) has a mass similar to Jupiter, so the transit should be readily detectable if it was taking place, while the third planet (designated c) has a mass intermediate between Saturn and Neptune. To my knowledge, no limits have been placed on transits of planets f (in the habitable zone) or d (out beyond the ice line), but with wider orbits the alignment has to be even more precise.

  26. John Sandlin

    I imagine the planet originally had more material and larger radius – but with that heat the lighter and more volatile materials (and what wouldn’t be volatile at almost 5000 degrees Fahrenheit) evaporated off and left the denser stuff behind.

  27. Two party-pooping observations:

    * A planet at 2700C would be glowing like a tungsten filament – and thus it wouldn’t make as deep an eclipse shadow as a black disk. Perhaps this makes the eclipse dip look shallower than it really is, meaning the planet is larger than we think.

    * I’m not sure there’s enough common heavy metals even in a ‘boiled off Jupiter’ to leave a ball of 8 Earth masses of lead/osmium/uranium etc. Far more likely is a giant ball of liquid nickel-iron with a nickel-iron atmosphere – at about 7-8g/cm3 that’s not too far off the computed density.

  28. My skiffy background, let me show you it… I once wrote a story involving a short-period (but not this short!) and tidally-locked world with unusually-strong gravitational and magnetic fields, that turned out to have captured a miniature black hole that was chewing it up from the inside. This real-science story made me remember it all over again.

    I dunno if my story is more or less plausible than a world with the specific gravity of lead and hot enough to melt same… ain’t science grand?

    — Steve

  29. hdhondt

    One thing that’s been going through my mind since we started finding exoplanets is how wrong our ideas about planet formation were. It was thought that rocky planets would form close to the star, and gas giants far away from it. Instead we’re seeing weirder planets every day, in orbits that would have been considered impossible even 10 years ago.

    While it may be too early to tell, it seems to me that the factor in the Drake equation (ne) which indicates the percentage of planets capable of supporting life, must be very small indeed. Hence to me the probability of finding intelligent life out there is very remote.

    But, I’m all in favour of investigating further. Any sign of life at all would be extremely welcome.

  30. @13. Nobody Important Says:

    55 Cancri (although I prefer the less-accurate Rho Cancri) is my very favourite star-system.

    Me too! ;-)

    I prefer Rho (or Rho-1) Cancris too. Shouldn’t that name take priority being older and higher in the nomenclatural scheme of things?

    It’s just so…familiar, somehow. But I wonder if 55 Cancri e is what’s called a “Cthonic planet”, a kind of planet that is just the leftover core of a gas giant.

    Absolutely! That’s one of the theories for it that makes a lot of sense. :-)

    Which has me wondering what a metallic hydrogen world would be like – how long could the metallic hydrogen stay in that form if the atmosphere is boiled away and what would the density of that be? Could that fit the observations for Rho-1 Cancri e too??

  31. @27. Terry Wiley :

    Two party-pooping observations:
    * A planet at 2700C would be glowing like a tungsten filament – and thus it wouldn’t make as deep an eclipse shadow as a black disk. Perhaps this makes the eclipse dip look shallower than it really is, meaning the planet is larger than we think.

    Perhaps so but a trio of possible party-poopers on your party poopers there :

    Firstly, there’s the question of an atmosphere which could block the light from the glowing surface beneath if its thick enough and composed of the right material. The hottest exoplanets ever found HD 149026 b is also the blackest absorbing nearly all the energy it recieves! Ash – hydrocarbons – and burnt material tends to be dark quite often after all!

    Secondly, thinking carbon – could this actually be one of the theoretical “Carbon Planet” class with a surface of asphelt-y, tarry material – and a layer of diamond deeper down?

    Thirdly and finally, contrast is a big part of transit eclipses isn’t it? Venus is very reflective – the brightest planet in our solar system, I think. However, against the backdrop of our Sun it looks like a dark dot crossing the far brighter face of our daytime star. Wouldn’t something similar apply here with 55 Cancri e?

    * I’m not sure there’s enough common heavy metals even in a ‘boiled off Jupiter’ to leave a ball of 8 Earth masses of lead/osmium/uranium etc. Far more likely is a giant ball of liquid nickel-iron with a nickel-iron atmosphere – at about 7-8g/cm3 that’s not too far off the computed density.

    Could well be but so much speculation – as are my suggestions and most others also. We have a few observed facts and a few ideas that could fit these facts but really, so little is known. We have no analogue to these sort of worlds in our solar system, and the one thing we’ve found about exoplanets is they keep being ever stranger and more exotic than we’d thought. Who knows what it’s like in reality – if only we had FTL starships already to go see! ;-)

  32. Messier Tidy Upper

    @23. amphiox :

    (Gases are worse, of course. Until we started looking, it was widely thought that the radius of Jupiter was about the largest radius you could make a planet/brown dwarf — piling on more matter would just compress it more than add to the radius. The ‘puffy’ planets — planets 2 or 3 times Jupiter’s radius — are really interesting for that reason.) A bit of an aside here, but this means that a super-jovian gas giant planet that isn’t puffed out by some other force with say 8-12J mass and the 1J radius would be substantially denser than lead. So the densest gas giant planets in the universe are going to be denser than the terrestrials!

    From what I understand, the density “heavyweight champion” exoplanet found so far is
    HAT-P-2b, a Hot Superjovian exoplanet weighing in at 8.2 Jupiters and as dense as Earth even though its mostly made of hydrogen. A 60 kg person would weigh 952 kg and experience 14 times Earth’s gravity at its cloudtops. HAT-P-2b orbits in 5 days around HD 147506 which is located 440 light years away in the constellation of Hercules.

    Although that may be out of date and, of course, I could be mistaken.

    And taking this consideration even further, the very largest brown dwarfs, at 79-80J in mass, again in a 1J radius, and the very smallest red dwarf stars, at 80-81J mass in 1J radius (radiant pressure from hydrogen fusion would puff out stars again beyond this mass range, but at the very borderline this effect would presumably be small and the object still remain close to 1J in radius), are going to have densities around 100, about 10X that of lead! Which should make them the densest normal (not degenerate like the superdense stuff of white dwarfs, neutron stars and black holes) matter objects in the universe….

    Well, the brains of some people – eg. many in politics, Creationists and Conspiracy Theorists – could perhaps give these a run for their money on that score! ;-)

    Although, yeah, that’s my understanding too regarding the densest ordinary matter objects being some red & brown dwarfs. :-)

  33. Gary Ansorge

    As has already been noted;

    From Wikipedia:Lead

    Boiling point 2022 K, 1749 °C, 3180 °F

    So, the atmosphere would likely have vaporized lead in it, though it might not be ONLY lead(ie,mercury, or other low melting temp metals). Probably not a good place for carbon based life forms(sic).

    Now, if we just had the tech,,,what a mining colony that could be.

    Of course, it might just have a neutronium core.

    Gary 7

  34. Mark

    The planet is zipping around its sun at 1,000,000 km/hr; 0.1% the speed of light.

  35. NitricAcid

    To have that density at that temperature, we’re not talking about lakes of liquid lead. We’re talking about an atmosphere of lead, copper, silver, and iron, over oceans of platinum and iridium. The mercury and zinc would have boiled away.

  36. andy

    Of course, it might just have a neutronium core.

    Formation scenario please, including compatibility with the existence of the outer planets.

    Really I don’t get why people want to jump to exotic scenarios when the mass/density values are perfectly compatible with a rock/iron planet. Yes the compression factor is somewhat greater than that for the Earth but 55 Cancri e is a more massive planet, hence this is expected.

  37. Messier Tidy Upper

    @ ^ andy : I think (#33) Gary Ansorge was joking there. Wish folks would remember their ;-) emoticons for clarity’s sake!

    Otherwise, yeah, you’re right. Lots more mass in relatively small volume = greater density, not such a mystery. Possibly higher metal levels such as a larger metallic core analogous to Mercury’s also makes some degree of sense too.

  38. Niall

    Ok, about the dinner plate..

    I think you’d want to hold it a lot closer to yourself, after all the further away you hold it, with gravity there being umpteen times more gravitationy, your outstretched arm is going to tire real quick?

    But then closer up your plate isn’t going to cast a big enough shadow… although my Mum has a turkey-sized roasting dish that might stop the whole ‘bursting into flames’ thing?

    (Sometimes I don’t think the survival advice offered in these pages is fully thought through.)

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