Cosmic Diary Marchis
What began as a tantalizing rumor has just become an astonishing fact. Today a group of thirty-one scientists, led by Guillem Anglada-Escude at the Queen Mary University of London, UK, announced the discovery of a terrestrial exoplanet orbiting Proxima Centauri. The discovery of this planet, Proxima Centauri b, is a huge breakthrough not just for astronomers but for all of us. Here’s why.
Astronomers know that exoplanets, or planets in orbit around stars other than our own, are numerous in our galaxy. NASA’s Kepler Space Telescope, which spent four years staring almost continuously at a tiny, distant patch of the sky, proved that there are on average two exoplanets per star in the Milky Way. That’s a lot of worlds! Kepler has also shown that most of our galaxy’s planets are indeed terrestrial—that is, like Earth or even bigger (so-called “super-Earths”). Unfortunately, the exoplanets discovered by Kepler are very distant—too far away for us to observe (and thereby study) using techniques available to most scientists today.
But Kepler is not our only tool for finding exoplanets. We can also use the radial-velocity method, which involves looking for the wobble a planet induces in its host star. This is an effective way to determine the mass of the planet and is not used only for distant stars. In the year 2000, for example, two astronomers announced the discovery of Epsilon Eridani b, a Jupiter-like exoplanet that we now call Aegir, orbiting around Epsilon Eridani, a bright star that’s only 10 light-years away. It was, until today, the closest exoplanet known. Now we now of one that’s far closer.
We also know of 3,374 exoplanets, an enormous large number, given that we discovered the first one only in 1995. Like the cartographers of the seventeenth century, who slowly build a map of our world, astronomers are drawing a map of our galactic neighborhood. We think we have a good handle on the location of nearby stars—that is, ones that are less than 50 light-years away. We know their distance size, temperature, and if they are multiple systems or single stars, for example; but ultimately what we would really like to add to this 3D map of the galaxy are the planets in orbit around these stars.
The Pale Red Dot group was particularly interested in finding planets around Proxima Centauri, the star closest to the Sun. Proxima Centauri is only 4.25 light-years away, so it’s literally in our cosmic backyard. Because of its small mass, it’s too faint to be seen with the naked eye, and was discovered only in 1915. At the end of the 1990s, astronomers tried to detect potential large planets in orbit around this star using the radial-velocity method and came back empty-handed.
In the article published today in Nature, a group of modern astronomers reported on what they learned by using two high-precision radial-velocity instruments: HARPS at the 3.6m telescope of La Silla and UVES at the VLT 8m class telescope, both part of the European Southern Observatory. Several of these observations were done as part of other programs that took place between 2000 and 2016, but from January 2016 to March 2016, the team collected what we call high-cadence data, a fancy way to state that the star was observed once per night to increase its chance of detecting a tiny variation in its motion o (~1 m/s, or the speed of a human walking) that might be caused by the presence of a small planet.
This ambitious program has paid off beyond our wildest dreams in that we have now unambiguously detected a planet with a minimum mass 1.3 times that of Earth orbiting the star right in the middle of the goldilocks zone (0.05 AU). I am not a specialist in RV measurement, but this detection seems quite convincing in that it has a false-alarm probability of less than 0.1% and uses a careful comparison of star activity (done by using additional small telescopes during the survey) that are known to mimic the signal of a planet. That is a very significant new data point to add in our cosmic map.
Did we find a terrestrial planet?
We don’t know for sure. The planet’s MINIMUM mass is 1.3 x Mearth since we don’t really know the orientation of the orbital plane with respect to the observer (RV provides a measurement of m sin i, with i the inclination of the system with respect to us). Assuming random orientations of orbital planes, we have a 90% probability that the true mass is < 2.3x the minimum mass, so 3x Mearth. In short, this could be a super-Earth or something more exotic, like a baby-Neptune.
Have we found a cousin of Earth?
Not yet. We don’t know the composition of the planet—keep in mind that we haven’t seen it but only its effect on its star. Consequently, we don’t have much information on the planet itself, but we do have a constraint on its mass (see above) and its orbit (one year of Proxima Centauri b is 11.2 days). Since the planet does not transit its star, we don’t know yet its size, hence its density.
Can life exist on this planet?
The planet is at the right place to have a temperature that allows the presence of liquid water on its surface. The question of habitability is however very complex. We need to confirm that this is a terrestrial planet. The best way to do that would be to directly image the planet using the giant telescopes equipped with extreme adaptive optics that are currently being built (i.e., the E-ELT, TMT, GMT). The angular separation between the star and the planet is 39 milli-arcsec, so a telescope as large as 30 m could resolve the system with the right instrument, detecting the planet and possibly giving us insights into its composition.
Can life thrive on this planet?
This planetary system is different from ours. Proxima Centauri is a M-type star that is known for sporadic flares, or outbursts of energy. Those luminous UV and X-ray flares could have sterilized the surface of the planet regularly and/or ejected a significant part of its atmosphere into space. The authors briefly discussed the possibility of habitability given the possible present of an extreme environment. I am betting that several follow-up papers on this topic will be published very soon. Astrobiology has taught us that life on Earth is resilient and can be found in extreme environments like deep oceans or protected from UV light in underground caves, so the possibility of a life somewhere in Proxima Centauri b cannot be rejected.
Ultimately, this discovery is a significant step on the road to mapping our galaxy. And it has given us a new world to explore, and one that is not too far away. We may not go there any time soon, but it will motivate us (and our funding agencies!) to design and build instruments to image and characterize this planet. What could possibly be more exciting than, in the not-too-distant future, get a picture of a terrestrial planet whose atmosphere we can see and on which we could possibly detect signatures of life? That monumental moment may come in the next decade, and will definitely happen faster now that we know where to point our telescopes.
Let me close by saying that it is astonishing that we were able to detect this small planet after only three months of observations. The Pale Red Dot group is planning similar campaigns for other nearby stars. In the future, for example, we may know if Alpha Centauri A and B, another nearby system composed of two stars almost identical to the sun, host a true cousin of the planet we call home.
We have no way of knowing where this quest will go, or when, or what it will find. But clearly, this could be the most astonishing journey in the history of humanity.
Another attack this month, this time in Nice in the middle of the 14 July celebrations. First, the obvious: this attack, which targeted people and families watching fireworks, was despicable and inhuman. Of course, I send my thoughts to the victims and their families as well as to my colleagues and friends who live in Nice. But I am tired of being an impotent witness to this outbreak of violence everywhere in the world, and I absolutely refuse to remain silent. Like many people, I feel like I’m living a nightmare, in a polarized world in which a tiny minority of violent sociopaths want us to embrace ideas from another time whatever the cost to us, our loved ones, our values, and our dignity as human beings. I hope that this stops and that we find a solution and learn, once and forever more, to live together in peace. But we are living in a very difficult time and we must prepare for many setbacks and tragedies.
The events in Nice also make it imperative that we learn certain key lessons. We, the citizens of free nations going about our daily business, are paying a huge price for wars launched in the first decade of this century by radical and ignorant politicians who manipulated their nations into engaging in a war without end, a war that accomplishes nothing except awakening extremism and hatred in the minds of the weak, the ignorant, and the alienated.This year and next, we can begin to correct this colossal and tragic mistake when we exercise our precious freedom to vote for a government of our own choosing. When we make that choice, we must reject divisive candidates who thrive on the poison of isolationism and xenophobia. Together, we must choose peace, unity and brotherhood. This is what we owe not only to ourselves and our loved ones, but also to the hundreds of victims of these attacks—the men, women, and children; the blacks, Latinos, Asians, and whites; the straight and gay, old and young; the Americans, French, Belgians, Turks and many, many others. They were all human beings whose lives suddenly and tragically ended during a summer evening’s fireworks display, or on the way home from a long trip, or while going to work, or while simply listening to music or enjoying the company of loved ones. Remember them when you vote. Remember them when you hear hatred spewed by radical politicians. Remember above all not to succumb to the easy path of stupidity and violence. We may be weary—but we are always stronger when we act together to fight for human freedom and dignity.
This is what the people of France knew in 1789. This is what the people of the United States knew in 1776. This is what the people of the Soviet Union knew in 1991. And today we must know in—our bones and in our hearts—that this is our only hope for turning tragedy into triumph.En Francais
Encore un attentat ce mois ci, cette fois à Nice en plein 14 juillet visant des familles qui regardaient un feu d’artifice. Je vais écrire l’évidence, dire combien cette attaque est abjecte et inhumaine, j’envoie bien sûr mes pensées aux victimes et à leur famille ainsi qu’à mes collègues et amis qui vivent à Nice. Je suis néanmoins fatigué d’être le témoin impuissant de ce déchaînement de violence partout dans le monde et je refuse de rester silencieux. Comme beaucoup j’ai l’impression de vivre un cauchemar, dans un monde polarisé dans lequel une petite minorité d’imbéciles violents veulent à tout prix nous faire accepter leurs idées d’un autre temps quelqu’en soit le prix humain. Je souhaite que ça s’arrête et que nous trouvions une solution pour de nouveau apprendre à vivre ensemble. Cela va être très difficile et il faut se préparer à des déconvenues et des difficultés.
Néanmoins nous devons tirer les leçons de ce qu’il s’est passé. Nous payons les effets secondaires des guerres modernes qui ont été initiées dans les années 2000 par des politiciens radicaux et ignorants qui ont manœuvré les opinions pour se lancer dans une guerre sans fin, une guerre qui ne mène à rien à part éveiller l’extrémisme dans les têtes les plus faibles.
Cette année et l’année prochaine beaucoup d’entre nous irons voter. Nous aurons alors à choisir entre des candidats diviseurs qui déverseront le venin de l’isolationnisme et de la xénophobie. J’espère que tous ensemble nous choisirons la politique de la paix, de rassemblement et de fraternité. Nous leur devons bien ça aux centaines de victimes de ces attentats, aux femmes, enfants, à ces hommes, blacks, latinos, blancs, straight, homosexuels, vieux, jeunes, Français, Américains, Belges, Turques… Ces êtres humains dont la vie s’est arrêtée un soir d’été en regardant un feu d’artifice, en rentrant d’un long voyage, en allant travailler, ou en écoutant de la musique. Pensons à eux lorsque nous irons voter. Pensons à eux lorsque nous écouterons les discours de haine de politiciens radicaux et ne nous laissons pas succomber à la bêtise et la violence. Nous sommes épuisés mais nous sommes toujours plus forts tous ensemble.
C’est ce que nous avons appris en 1789 en France, c’est aussi ce que les habitants des colonies anglaises d’Amériques on comprit en 1776, et plus récemment ceux qui ont brisé les chaines de l’Union Soviétique en 1991. Aujourd’hui nous devons garder en tête que tous ensembles nous pouvons transformer une tragédie en triomphe.
(87) Sylvia was discovered in 1866 by N.R. Pogson, a British astronomer located in Madras, India. This main-belt asteroid is large with a diameter of ~150 km. That’s all we knew until recently.
In 2005, we discovered two moons around the asteroid that we baptized Romulus and Remus, sons of the Rhea Sylvia and founder of Rome. Both moons are very small with a diameter estimated to ~20 and ~7 km. The primary is irregular with a diameter of 193 x 129 x 124 km. Both moons orbit around the primary describing a coplanar and almost circular orbit.
From a long term study of their orbit we concluded that the primary is possibly differentiated, made of a dense core of ice. How the moons have formed is still unclear. They could be the product of a catastrophic disruption of a large asteroid early in the history of the solar system.
(87) Sylvia is the first triple asteroid ever discovered. I have a fond memory of the day, I presented this discovery at the ACM conference in August 2005 at Buzios, Brazil, simultaneously with the publication in Nature. It was a very special moment in my career.
Today we know six triple asteroids in the asteroid main-belt (45 Eugenia, 87 Sylvia, 93 Minerva, 130 Elektra, 216 Kleopatra, 3749 Balam)and there are probably more of them waiting to be discovered with the next generation of space-based and ground-based telescopes.
Another day, another video!
This time I am posting a video of the binary L5 Trojan Asteroid (617) Patroclus-Menoetius. In collaboration, with the team at the California Academy of Sciences, we have created a model of this interesting binary asteroid system which shares its orbit with Jupiter.
In 2001, a group of astronomer discovered that the L5 Trojan asteroid (617) Patroclus is in fact made of two components. In 2006, using Laser Guide Star Adaptive Optics System at W.M. Keck Observatory, we showed that those two components orbit around the center of mass of the system in ~4 days at 680 km describing a circular orbit. We named the second component Menoetius, the argonaut father of Patroclus in the greek mythology.
From the estimate of the size (derived from various techniques including Spitzer observations of mutual events taken in 2010 and stellar occultation on October 2013), we found out that the components are less dense than icy water, with a grain density very close to satellites of giant planets (like Amalthea, moon of Jupiter).
Because Patroclus has a different color and density than (624) Hektor, we speculated that it could be a captured Jupiter-Saturn asteroid which ended up in the gravitational well of the Sun-Jupiter system during the migration of the giant planets 3.7 Billion years ago. Its binary nature could be the result of tidal disruption when primitive asteroid had a close encounter with Jupiter before the capture.
Ultimately, we will need to send a spacecraft there to really understand this system. NASA has pre-selected the LUCY new discovery mission which could flyby this binary Trojan asteroid in 2033.
I finally started uploading some of the animations of the talk that I gave last month at the California Academy of Sciences. Today let’s watch (624) Hektor, the binary and bilobed largest Jupiter-Trojan asteroids. This is a puzzling multiple asteroid system with a lot of mysteries (eccentric and inclined orbit of the moon, complex shape and structure for the primary, …).
Our study based on AO observations collected over 8 years was published in 2014. The conclusion of our work is that 624 Hektor is probably a captured Kuiper-belt object and the moon formed a long time ago from the slow velocity encounter of the components.
We will probably need to send a spacecraft over there to really understand this complex mini-geological world. The good news is that several space agencies, including JAXA and NASA, are thinking about that.
I would like to thank to my colleague Josef Durech, Matija Ćuk, Julie Castillo, Frederic Vachier, Jerome Berthier and numerous more for their long-term contribution to this project. I also should include my sister Helene Marchis for making the first drawing of this system. Thanks as well to the California Academy of Sciences for making those great CGI videos and the director Ryan Wyatt for inviting me.