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A Star With Nine Planets, Maybe More?

The views expressed are those of the author and are not necessarily those of Scientific American.

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Planet, planet, planet.... (Image credit: ESA)

Exactly how many planets orbit any given star is still a major unknown in exoplanetary science. The two primary techniques for detecting planets and quantifying their characteristics have significant limitations that blinker us to the full contents of other solar systems. Radial velocity measurements pick up the tell-tale motion of a star around a system’s common center-of-mass, or balance point, due to the gravitational pull of any planets. But the smaller the planets and the further they are from the star the weaker the signal. Multiple planets and longer orbital periods confound the situation by producing complex patterns that may also be incompletely sampled in data that spans only a few years. Transit observations, such as those undertaken by the Kepler mission, are biased towards the detection of large planets in small orbits around small stars where it is most likely for a planet to block the light from the star more frequently.

Spot the planets - if you can. Time series data of the radial velocity of HD 10180 (Lovis et al. 2011)

All of this means that in essentially all currently known systems we may have only incomplete information about the true number of orbiting planets. Nonetheless, stars with multiple planet detections certainly crop up. Of the over 550 confirmed exoplanetary systems there are over 90 with more than one planet (a total of more than 760 worlds). Now a new study of radial velocity data from the HARPS instrument suggests that one of these systems, HD 10180, may harbor nine major planets – usurping our own solar system from the top of the pile of planetary richness.

In a recent paper Mikko Tuomi applies a sophisticated Bayesian (probabilistic) analysis technique to measurements taken over a period from 2003 to 2009. The star HD 10180 is a close solar-analog, about 6% more massive than the Sun and of similar composition it lies some 127 light years from us. Previous investigations of this data had suggested that there could be six or perhaps 7 planets in the system, Tuomi claims the presence of two more objects, albeit at relatively low statistical significance. The masses of these worlds ranges from more than about 1.3 times the mass of the Earth to more than 65 times the mass, and the orbits place all of them within the equivalent of Jupiter’s orbit in our solar system.

If further observations bear out this claim, it will make the HD 10180 system an extraordinarily busy place. Nine planets, ranging from possibly Earth-sized to Neptune-sized and larger, all crammed into this star’s inner realm. Previous studies of other systems have already suggested that planets are often ‘maximally packed‘ – if they can form they will and as close together as allowed by gravitational dynamics. In such cases, any reduction in orbital spacing between worlds or increase in their masses would make the system unstable. In the case of HD 10180, the computer simulations that will tell us if these 9 planet candidates can actually form a stable system over billions of years have yet to be run, but it looks promising.

All of which raises an even more interesting question. There is no obvious reason for these to be the only planets in the HD 10180 system. The fact that the outermost candidate appears to have a 6 year orbital period is a strong function of the limitations of the current set of measurements, and the packed nature of this system doesn’t exclude even more objects on larger orbits. In other words, there is no telling how many planets actually orbit further out around HD 10180. The total mass of the 9 candidates is at least 170 Earth masses, but this is just over half the mass of Jupiter, and even if it were double this, it suggests that the original proto-planetary disk that made these planets could have had plenty of additional material to make more (assuming some similarity to our own solar system).

Just as we’ve seen that the galaxy is rife with planets, making our own circumstances seem a little less special, it may be that our own planetary richness is also quite mediocre.


Caleb A. Scharf About the Author: Caleb Scharf is the director of Columbia University's multidisciplinary Astrobiology Center. He has worked in the fields of observational cosmology, X-ray astronomy, and more recently exoplanetary science. His books include Gravity's Engines (2012) and The Copernicus Complex (2014) (both from Scientific American / Farrar, Straus and Giroux.) Follow on Twitter @caleb_scharf.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. RCWhitmyer 4:49 pm 04/10/2012

    Another step away from the center of the universe.

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  2. 2. Metridia 8:47 pm 04/10/2012

    Any estimate on the age of this star? Didn’t the early Solar System similarly have a lot of packed-in planets?

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  3. 3. Caleb A. Scharf in reply to Caleb A. Scharf 9:20 pm 04/10/2012

    The age estimate for HD 10180 is about 4.3 billion years plus or minus 0.5 billion, so very similar to our own solar system.

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  4. 4. 7:40 am 04/11/2012

    To Celeb A Celeb A Scharf

    Our solar system was formed about 4.5 billions years ago. HD 10180 is located 127 light years and and its age is 4.3 billions years i.e. in the same range as our Sun. Within this distance of 127 light years, millions of stars shall be in position. Any idea Or study regarding nos. of stars within this length? Will the age of all these stars also in the range of 4.5 billion years? If the age of all these stars is also in this range, can we suspect that all these stars were formed from same nabula under a common process ?

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  5. 5. And Then What? 8:05 am 04/11/2012

    What has always amazed me, and probably will amaze me until the day I die is; why would a reasonably intelligent person think our situation is somehow unique? The fact that we may have discovered a similar system, at least in planet numbers, should not surprise anyone. Our solar system formed due to the law of Gravity that appears to be Universal, at least as far as we can tell on a galactic scale. So only blind arrogance, borne of an equally blind ignorance, would be the only thing that would make someone think our circumstance is somehow a “special case” in the vastness of the Universe. Finding a bunch of barren rocks flying around some other Star, while nice, is certainly not earth shattering news. Or at least it should not be. Having said that I do think that continuing to search is a worthy endeavor since I am convinced, by my belief in the power of Logical thought, that one day, if we don’t succeed in our efforts to kill ourselves off, we will find we have a lot of neighbors, but keep in mind that “good fences make good neighbors” will probably still apply.

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  6. 6. Caleb A. Scharf in reply to Caleb A. Scharf 8:22 am 04/11/2012

    vinodsehgal1957 – there certainly are many stars within 127 light years of us, but not millions, more in the range of 20,000-40,000. It’s an excellent question as to their age and similarity to the Sun, the bottom line is that they cover a wide range of ages. The question of where our stellar birth cluster is/was remains a pressing one (John Matson had an excellent post at Sci Am on precisely this a few weeks ago) – the Sun has either been ejected from that original cluster (which may be somewhere within a couple thousand light years of us) or the cluster itself has dissociated, pulled apart by galactic gravitational tides as we orbit the galactic center once every 210 million years. So right now I guess the way to think of it is that we’re surrounded by strangers!

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  7. 7. JHewson 8:26 am 04/11/2012

    are we sure it’s not some kind of reflection of our own system?

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  8. 8. Caleb A. Scharf in reply to Caleb A. Scharf 8:32 am 04/11/2012

    I agree that the notion of us as ‘special’ in any way can seem ridiculous. However, it’s important to recognize the complex history of thought behind questions of our true significance or insignificance. Even Copernicus still felt the Earth was ‘special’, in fact he wrote that a heliocentric arrangement placed us ‘at the center’ of the orbits, between Sun, Mercury, Venus and Mars, Jupiter, Saturn (those being the only planets he knew about). With a more modern perspective, while the principle of mediocrity (or cosmological principle) states that we are neither unusual in our place or in our view of the cosmos, and has been enormously important in driving the science of quantitative cosmology, it’s also pretty obvious that we *do* actually occupy a very particular (albeit not necessarily *special*) place in the universe – a certain position in a certain type of galaxy at a certain cosmic time and with a certain type of planet beneath our feet etc etc. And so the big scientific question to be asked is ‘which of these factors really matters?’ Because we want to know precisely how common or unusual they are. Thus, learning about things like the efficiency with which planets are formed around particular stars is actually critical.

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  9. 9. 2:23 am 04/12/2012

    To Celeb A Scharf

    Ref; Your comments at 6 and 8

    1.0 I had read that Article by John Matson which indicated our Sun to be an alien in our Solar system. However, I think, if a detailed study is carried, in the vicinity of our solar system, of the stars which are similar to our Sun in 1) Age 2) Mass 3) Composition, our knowledge regarding formation of our Sun and Solar System may become more evidence based.
    2) A you have mentioned that Sun (Along with Solar System) orbits around galactic center once around 210 million years.
    It is well accepted in the scientific fraternity that our Sun is located at about 26000 light years from galactic center, occupied by black hole Saggitarus A whose mass is about 4 million times the mass of Sun. And our Sun orbits around Saggitarus A at a speed of about 210 KM/sec.
    Can galactic Center, though having a black hole as massy as 4 million times the mass of Sun but it being also at a distance of about 26000years, provide adequate pull on our Sun to make it orbit around it at a speed of 210 KM/sec? Further, how the gravitational force from inside the black hole acts upon our Sun ? Does a black hole allows gravitational force( originating inside its event horizon) to act upon bodies outside its event horizon?
    3) Is the movement of Sun around galactic Center based upon theoretical amd mathematical predictions or this carries some empirical studies.
    4) In our Solar system, 99.86% of mass is concentrated at center in Sun and as per Kepler’s Law planets move around Sun in elliptical trajectories.A question which confuses to my mind is why in elliptical trajectories and why not in perfect circular one which appear to be more obvious choice?
    5)In case of our galaxy, situation does not appear similar to our Solar system. There are 200-400 billion of stars and plants and a large nos. of planets ( Recently an Article in SA mentioned that on average there are 1.6 planet per star in our galaxy). There may also be a nos. of black holes apart from central Saggitarus A. Therefore, mass at the center of our our galaxy shall be a very small fraction of the total mass of galaxy. In view of this:
    a) How gravitational pull is provided on our Sun and other stars to orbit these stars around galactic center? Is there any solid evidence that 200 billion stars are orbiting around galactic center
    b) Will a large system of 200-400 billion stars and equally planets moving around a common center shall be gravitational stable.
    c) Do stars also follow elliptical trajectories around galactic center? There being no comparison between galaxy and solar system, applicability of Kepler Law should not arise at galactic level.
    I understand that prediction of dark energy is based upon elliptic orbits of stars around galactic centre. If elliptic trajectories of stars are not established, will it not endanger the prediction of dark energy.
    It seems to me that during the past 60-70 years, though our knowledge of cosmology and astronomy has manifold increased, but this has also confounded the problems manifold and we are moving increasingly in the jungle of wilderness and darkness

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  10. 10. carlonne 4:49 pm 04/12/2012

    In response to those who comment that people on this planet still think we are the only one’s, consider the amount of mis information and out right religious ignorance that pervades this planet! Until there is a complete rejection of dogmatic organized religion and it is replaced with scientific answers, will we begin to realize that it’s near impossible that we are the only intelligent life out there.

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  11. 11. Postman1 11:26 pm 04/12/2012

    Caleb – Thanks for another good article.
    Question: Do we know if any of the super Earths in this system lie in it’s goldielocks zone? It is early in the study, but I thought we might have some idea. Personally, I think there is more likelyhood of life on large moons of gas giants in the zone, but it is all conjecture anyway.

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  12. 12. 10:55 am 04/13/2012

    There are 200-400 billion stars in Milky way galaxy only and estimates put galaxy nos. in observable universe at about 100 billion. As such, we can not fathom the vastness of universe by any thought process And within this vastness, we are roaming in our solar system for the presence of water at Mars or some other planet. As such, how humankind can unravel the mystery of our uniqueness in universe? Nevertheless, keeping into view the unimaginable vastness of universe, there appears low likelihood of our civilization on this planet to be unique one. There might be life(s) and civilizations in some other corners of universe, who may also be considering them as unique But who knows this? and whether we shall ever be able to know this?

    In Cosmology/ Astronomy and Astrobiology we have become obsessed with light signals and presence of water.

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  13. 13. scottryan1 4:49 am 04/15/2012

    Well once we build a Hi tec space ship, we have away to make instant contact to earth or other space ships, even if they are 1 billion light years away.

    How to communicate to space ships 1 billion light years away instantly.

    “quantum nonlocality and a phenomenon known as entanglement.”

    Brian Green explains this very clearly in his book “The Fabric if the Cosmos.”

    so we can use it in the future like sos.

    so if you make the 2 of them, you make 1 turn a bit for a letter or a word. this way we could communicate with a space ship 200 million light years away instantly?

    Well we could even get lasers to read it by how it turns and what we make a letter, to how many times it turns or moves left or right.

    well what it is, is a particle and so on, and when they are made, the other one moves the opposite way as the other 1 is moved. if 1 turns down, the other one will turn up.

    so the particles will move in a different detractions instantly to the partner other 1. as in it will not fly away, but rotate up down or right or left. you just have to move 1 of them, and the other 1 will move the other way, but instantly no matter how far apart.

    This is the future of long distance contact and more.

    so even over 10 million+++++ light years, the other one will turn up or down, to the left or right a bit, instantly no matter how far apart.

    Well also satellites can use it 2. this way data can get back instantly.

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  14. 14. Caleb A. Scharf in reply to Caleb A. Scharf 3:36 pm 04/15/2012

    Postman1, I believe that Tuomi finds that there is ‘room’ in the orbital configuration (while retaining dynamical stability) for a lower mass (i.e. Earth mass or lower) object in the nominal habitable zone of this system.

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  15. 15. 8:42 am 04/16/2012

    To Scottrayn1
    I am not rebutting your wishful sc-fi thinking. But tell me if a particle can at two locations, separated by distance of 1 billion light years, at the same time. If is so, is there any empirical experimental evidence for the same? If it is so, will it not be in violation of Einstein’s theory of SR.

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  16. 16. Dileep Sathe 9:13 am 04/23/2012

    I think this question can not be answered affirmatively because for finding the expression of orbital velocity of a planet we simploy equate the gravitational force, GMm / r^2 with the centripetal force, mv^2 / r. But we do not put any condition either on mass or distance. So any planet, moving on an elliptical path as required by Kepler, can have any mass any distance – we just have to equate the gravitational force with the centripetal force and get the expression of orbital velocity.

    Still then, this is a good question because it has direction logical connection with the new definition planet, adopted by the IAU in 2006, and my objection to it – IAU46, NL 66, 28 March 2007. For easy debate, write me directly on

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