Red Dots: The Search for Nearby Earth-Size Exoplanets

On August 24, 2016 an international team of astronomers led by Guillem Anglada-Escudé (Queen Mary University of London) announced the discovery of Proxima Centauri b – an Earth-size planet orbiting inside of the habitable zone (HZ) of the closest known star to our solar system, Proxima Centauri. While the existence of this exoplanet had been suspected for years based on periodic variations observed in the star’s radial velocity (RV), it took an intensive data collection campaign called the Pale Red Dot involving observatories around the world to gather the data required to verify its existence. The Pale Red Dot campaign’s nearly simultaneous measurements of the RV of Proxima Centauri gathered during the first quarter of 2016 by the HARPS (High Accuracy Radial velocity Planet Search) team working at the European Southern Observatory (ESO) and complementary multiband photometric measurements from other observatories allowed astronomers to differentiate the effects of an orbiting exoplanet from those of stellar activity (see “Habitable Planet Reality Check: Proxima Centauri b”).

Dr. Guillem Anglada-Escudé is shown during the ESO press conference in Munich, Germany during the announcement of the discovery of Proxima Centauri b on August 24, 2016. (ESO/M. Zamani)

After the stunning success of this campaign, Guillem Anglada-Escudé and his team have been able to secure yet another block of valuable observation time on telescopes around the globe for a second, now expanded campaign which started in June 2017 called Red Dots. In addition to observing Proxima Centauri to search for more exoplanets, two more nearby red dwarf stars have been added to the survey: Barnard’s Star and Ross 154. This new project to search for nearby exoplanets nicely complements the partnership ESO announced on January 9, 2017 with the Breakthrough Initiative which which aims to demonstrate proof of concept for a new technology that will enable ultra-light unmanned spaceflight at 20% of the speed of light. Such nanocraft could be sent to exoplanets found in ESO searches of nearby star systems such as α Centauri.

The properties of the Red Dots targets are summarized below in Table 1. The properties for Proxima Centauri are taken from Anglada-Escudé et al. while those for Barnard’s Star and Ross 154 are taken from Bonfils et al. as well as other sources.

Table 1: Properties of Red Dots Targets
Name Proxima Cen Barnard’s Star Ross 154
Gliese Number 551 699 729
Spectral Type M5.5V M4V M3.5V
Surface Temperature 3050 K 3134 K 3340 K
Mass (Sun=1) 0.12 0.16 0.17
Radius (Sun=1) 0.14 0.20 0.24
Luminosity (Sun=1) 0.0015 0.0035 0.0045
Rotation Period (days) ~80 ~130 ~3
Distance (LY) 4.24 5.94 9.68

 

Here is some historical background on the targets of the new Red Dots campaign and what earlier searches for exoplanets have found.

 

Proxima Centauri

Proxima Centauri is a V magnitude 11.1 red dwarf located, as its official IAU designation would suggest, in the southern constellation of Centaurus. This star was discovered in 1915 by Scottish astronomer Robert Innes (1861-1933) while he was the director of the Union Observatory located in Johannesburg, South Africa. Using photographs taken in April 1910 and July 1915 using a 25-cm astrographic telescope built by the pioneering English astrophotographer John Franklin-Adams (1843-1912), Innes demonstrated that the dim star shared a common proper motion with the pair of much brighter Sun-like stars in the nearby α Centauri system which has recently had its official IAU designation changed to Rigil Kentaurus. Parallax measurements by Innes and others published in 1917 convinced him that the star was actually slightly closer to us than α Centauri AB leading Innes to name it “Proxima Centauri”.

Scottish astronomer Robert Innes discovered Proxima Centauri in 1915 while working at the Union Observatory in South Africa. (South African Library)

Subsequent work by other astronomers in the decades to follow confirmed that Proxima Centauri was and still is the closest known star to our solar system by about an eighth of a light year. Because of its similar distance, shared proper motion and close proximity in the sky, it has been assumed for the last century that Proxima Centauri is gravitationally bound to α Centauri AB about 13,000 AU distant but proving it turned out to be more difficult than expected given Proxima’s distant orbit. It was not until recently that Pierre Kervella (Unidad Mixta Internacional Franco-Chilena de Astronomía/Observatoire de Paris), Frederic Thévenin (Observatoire de la Côte d’Azur) and Christophe Lovis (Observatoire Astronomique de l’Université de Genève) were able to prove definitively that all three stars were gravitationally bound. Using the latest data on the positions and motions of the stars as well as corrections for how precision radial velocity (RV) measurements are affected by various stellar phenomena, Kervella et al. were able to show that Proxima Centauri is locked in an eccentric orbit that ranges from 4,300 to 13,000 AU with a period of about 550,000 years (see “The Orbit of Proxima Centauri”).

This diagram shows the orbit of Proxima Centauri recently derived by Kervella et al.. Click on image to enlarge. (Kervella et al.)

With its distance established in 1917, it was also realized that Proxima Centauri had the lowest absolute magnitude of any star then known. After examining archived photographic plates, American astronomer Harlow Shapley (1885-1972) discovered in 1951 that Proxima Centauri is a UV Ceti type variable star which experiences occasional flares that can cause the brightness of the otherwise quiescent dwarf star to increase by several percent or much more for brief periods of time. More recent photometric studies of how the brightness of Proxima Centauri varies over time indicate that it has a rather long period of rotation on the order of 80 days. This slow rotation and its association with the extensively studied stars α Centauri A and B suggest that Proxima Centauri has an age of around five billion years – roughly the same as the Sun and our solar system.

Because of its small mass and relative closeness, Proxima Centauri has been the target of searches for substellar and planetary companions for decades. During the last quarter of a century, increasingly sensitive detection methods have been employed in this search capable of finding ever smaller companions and ultimately planets. However, by the opening decade of the 21st century, direct imaging and astrometric surveys looking for the subtle reflex motion of an orbiting object had failed to find anything eliminating the existence of any object larger than approximately Jupiter-mass (or MJ) objects with orbital periods greater than about 50 days (for a detailed summary of these earlier searches, see “The Search for Planets Around Proxima Centauri”).

A view of Proxima Centauri acquired by the Hubble Space Telescope. (ESA/Hubble/NASA)

By far the most sensitive searches to date have used precision measurements of the star’s RV to detect an orbiting unseen companion. While published results from RV surveys of Proxima Centauri go back over a decade and a half, among the most accurate RV results up until the last decade were based on the work of Michael Endl (McDonald Observatory) and Martin Kürster (Max Planck Institute) published in 2008. Endl and Kürster had observed Proxima Centauri as part of an ongoing program to survey 40 M-dwarf stars using UVES (Ultraviolet and Visual Echelle Spectrograph) on the 8.2-meter telescope designated UT2 – one of four such telescopes that make up the VLT or Very Large Telescope at the European Southern Observatory on Cerro Paranal in Chile.

Based on an analysis of the 229 spectra they obtained between March 2000 and March 2007, Endl and Kürster found no statistically significant RV signature which would indicate the presence of an orbiting extrasolar planet. Based on the detection limits of their data set, it was found that there were no planets orbiting Proxima Centauri with MPsini values of two times that of the Earth (or 2 ME) with an orbital period of 2 days and up to 20 ME for planets with orbital periods of nearly 2,000 days. Since RV measurements alone are incapable of determining the inclination, i, of the exoplanet’s orbit with respect to the plane of the sky, only the minimum mass or MPsini value can be derived with the true mass likely being larger.

Minimum mass upper limits as a function of period for planets in circular orbits around Proxima Centauri based on work by Endl and Kürster. The shaded area labelled “HZ” displays the approximate location of the conservatively defined habitable zone. Click on image to enlarge. (Endl and Kürster).

The UVES observation program described by Endl and Kürster was not the only survey to be making precision RV measurements of Proxima Centauri looking for orbiting planets. As part of their systematic survey of nearby M-dwarf stars, the European-based HARPS (High Accuracy Radial velocity Planet Search) team had been making precision RV measurements of Proxima Centauri using a purpose-built spectrograph attached to the European Southern Observatory’s 3.6-meter telescope in La Silla, Chile. But even after collecting over two hundred spectra of their own between May 2004 and January 2014, the HARPS team had detected only a hint of a variation with a period of about 11 days but with a false alarm probability still too high to be considered a reliable planetary detection. More data were needed.

Pale Red Dot was an international search for an Earth-like exoplanet around the closest star to us, Proxima Centauri, which employed HARP on ESO 3.6-meter telescope in La Silla, Chile (shown here) and other southern hemisphere observatories. (ESO/Pale Red Dot)

Over the next two years, Guillem Anglada-Escudé organized a dedicated, high-cadence observation campaign to look for this possible exoplanet (or exoplanets) orbiting Proxima Centauri which would eventually be known as the Pale Red Dot – a play on Carl Sagan’s reference to the habitable Earth as a pale blue dot. Between January 19 and March 31, 2016 a total of 56 new spectra were recorded using the HARPS spectrograph on 54 different nights as part of this campaign. In order to eliminate the possibility that any observed variation in RV was the result of stellar activity, nearly simultaneous photometric measurements were made at two cooperating observatories to track the brightness of Proxima Centauri and how it changed over time.

Anglada-Escudé et al. published the results of their Pale Red Dot campaign in the August 25, 2016 issue of the peer-reviewed science journal Nature. Fitting a Keplerian orbit to the combined HARPS and UVES RV data sets, Anglada-Escudé et al. found a planet with an orbital period of 11.19 days and a Doppler semiamplitude of 1.38 meters per second. Combined with the presumed properties of its host star, the new exoplanet, dubbed Proxima Centauri b, has a minimum mass or MPsini of 1.27 ME and mean orbital radius of 0.0485 AU – squarely inside most definitions of the habitable zone (HZ) (for more details about this discovery, see “Habitable Planet Reality Check: Proxima Centauri b”).

This plot shows how the radial velocity of Proxima Centauri and how it varied during the Pale Red Dot campaign of the first quarter of 2016. (ESO/G. Anglada-Escudé)

While the world was hailing this discovery of a potentially habitable exoplanet orbiting our nearest neighbor, there were hints in the data of more to come. According to the original discovery paper, residuals in the RV data after the effects of Proxima Centauri were removed had a semiamplitude of about three meters per second hinting at the presence of a second exoplanet with an orbital period currently estimated to be somewhere in the 40 to 400-day range. According to recent discussions, there are also hints of periodic signals with periods less than six days present in the data as well. Once again, more data are needed to characterize these signals and determine whether they are the result of stellar activity or the presence of additional planets orbiting Proxima Centauri.

 

Barnard’s Star

Barnard’s Star, whose long-time informal designation was officially adopted by the IAU on February 1, 2017, was named after American astronomer E.E. Barnard (1857-1923). Using the famous 40-inch (one-meter) refractor at the Yerkes Observatory, Barnard made a series of nine position measurements during June and July 1916 of what was later identified as BD +04° 3561 from the Bonner Durchmusterung astrometric star catalog compiled by the Bonn Observatory between 1859 and 1903. Comparing his visual observations with measurements from a photograph he took of the same region of the sky on August 24, 1894 when he was at the Lick Observatory, Barnard discovered that this star was moving at a rate of 10.3 arc seconds per year beating the previous record holder for the highest known proper motion, Kapteyn’s Star, cataloged in 1898 (see “Habitable Planet Reality Check: Kapteyn b”). Barnard also reported that a spectrum of the star taken by American astronomer Walter S. Adams (1876-1956) using the 1.5-meter reflecting telescope at the Mt. Wilson Observatory established this star as being a M dwarf. Later parallax measurements from Yerkes and other observatories showed Barnard’s Star to be the second closest star system after α Centauri with the distance currently pegged at 5.94 light years.

American astronomer E.E. Barnard discovered Barnard’s Star in 1916 while working at the Yerkes Observatory.

Because of its high proper motion, large parallax and the relative ease of observing it from major observatories around the globe due to its location in the constellation of Ophiuchus near the celestial equator, the V magnitude 9.5 Barnard’s Star has been a target of detailed investigation over the last century like no other red dwarf in the sky. The fast motion of Barnard’s Star through our part of the galaxy, its comparatively low concentration of “metals” (i.e. what astronomers call elements heavier than helium) and its slow rotation period of about 130 days all suggest that it is an old star with an age of about ten billion years. While ancient in comparison to the Sun, Barnard’s Star has completed only a tiny fraction of its lifetime on the main sequence which is estimated to be on the order of a trillion years. Despite its age, it has been observed to flare on occasion but it is considered a BY Draconis type variable whose modest changes in brightness is caused by starspots and other forms of surface activity modulated by the star’s slow rotation.

One of the best known observers of Barnard’s Star during the 20th century was undoubtedly Dutch astronomer Peter van de Kamp (1901-1995). From 1937 to 1972, van de Kamp was the director of Swarthmore College’s Sproul Observatory in Pennsylvania where he specialized in precision astrometry and became highly respected in the astronomical community for the quality of his work. A firm believer that planetary systems were common, van de Kamp pushed the limits of his equipment and measurement techniques to make not only precision measurements of the parallax and motions of nearby stars like Barnard’s Star, but also to search for the telltale wobble in the motion of a star that would indicate the presence of unseen companions.

Dutch astronomer Peter van de Kamp was probably the most famous observer of Barnard’s star during his tenure at the Sproul Observatory from 1937 to 1972.

In 1963 van de Kamp published the results of his analysis of the motion of Barnard’s Star since 1916 and claimed he found the wobble caused by an orbiting planet with a mass of 1.6 MJ and an orbital period of 24 years. This was followed in 1969 when van de Kamp published an updated analysis which now included an extra five years of new data indicating the presence of two planets with periods of 12 and 26 years possessing masses of 0.8 MJ and 1.1 MJ, respectively. While the discovery prompted much speculation and even led to this system to be selected as the hypothetical target for the Project Daedalus starship studied by the British Interplanetary Society during the mid-1970s, there was genuine skepticism about van de Kamp’s claim. Over the following decades, other observatories failed to find any evidence of these Jupiter-sized exoplanets and it is now believed that subtle instrument artifacts created a spurious planetary signature (for a detailed account of this and other attempts to search for planets orbiting Barnard’s Star, see “The Search for Planets Around Barnard’s Star”).

With over half a century of published results of searches for planets orbiting Barnard’s Star, by far the most sensitive and comprehensive to date is a 2013 paper with Jieun Choi (University of California – Berkeley) as the lead author. For starters, Choi et al. presented an analysis of 248 RV measurements of Barnard’s Star acquired between 1987 and 2012 using equipment at the Lick and Keck Observatories which showed no hints of van de Kamp’s putative exoplanets. Using the more accurate RV measurements taken with HIRES (High Resolution Echelle Spectrometer) at the Keck Observatory after August 2004 along with an independent data set of 226 published RV measurements acquired over six years using ESO’s UVES, Choi et al. failed to find any hint of orbiting exoplanets with periods out to 5,000 days. Based on a detailed analysis of the data set, Choi et al. found that for orbits with periods less than 10 days, 100 days and two years, planets orbiting Barnard’s Star with MPsini values greater than about 2 ME, 3 ME and 10 ME, respectively, can be excluded.

The detection threshold in terms of minimum mass as a function of orbital period based on an analysis of Keck radial velocity data by Choi et al. for planets with various orbital eccentricities, e. Click on image to enlarge. (Choi et al.)

In addition to these other surveys, the RV of Barnard’s Star has also been monitored by the HARPS team. An analysis of their initial survey results for M dwarfs with team leader Xavier Bonfils (Institut de Planétologie et d’Astrophysique de Grenoble) as the first author was published in 2013. A total of 22 RV measurements of Barnard’s star showed a clear trend over time with a weak periodicity of about 10,000 days and a shorter period of 3.1 days. Neither of these signals represent a reliable planetary detection but unpublished analyses of the historical HARPS, UVES and HIRES radial velocity measurements now suggest the presence of a super Earth size exoplanet in a distant “cold orbit”. As was the case with Proxima Centauri, more data are needed to verify the detection of any exoplanets.

 

Ross 154

The last target of the current Red Dots campaign, known as Ross 154, is a V magnitude 10.4 star located in the constellation of Sagittarius. Its common designation comes from being the 154th variable star cataloged by American astronomer Frank E. Ross (1874-1960) which first appeared as part of the fourth installment published in 1925. Ross had just accepted a position at the Yerkes Observatory in 1924 and had inherited Barnard’s collection of astronomical photographic plates allowing him to note changes in star brightness in addition to charting their motions. By comparing new plates with those from Barnard’s archive, Ross determined that Ross 154 had a noticeable proper motion which today is measured to be 0.66 arc seconds per year suggesting that it was relatively nearby.

American astronomer Frank E. Ross (shown here with the famous 40-inch Yerkes refractor in 1925) first cataloged Ross 154 in 1925. (Lick Observatory)

In 1938, American astronomer Walter C. O’Connell published the first parallax of Ross 154, which was identified as a M-dwarf for the first time in the same work as well. Measurements secured by O’Connell at the Southern Station of the Yale University Observatory in Johannesburg, South Africa established Ross 154 as the sixth closest stellar system then known. Because of its closeness, Ross 154 was included in the first edition of the Gliese Catalogue of Nearby Stars in 1957 earning its other common designation of GL 729 after the creator of the catalog, German astronomer Wilhelm Gliese (1915-1993). With the best current measurements placing this star at a distance of 9.68 light years, Ross 154 is still considered the ninth closest star system today when including the more recently discovered brown dwarf systems of Luhman 16 and Wise 0855-0714.

Ross 154 is a UV Ceti type flare star like Proxima Centauri with the first flare of 0.4 magnitude noted by American astronomer and pioneer in precision photometry, Gerald E. Kron (1913-2012), during a routine photometric survey conducted by the Lick Observatory in 1951. With flares observed increasing the brightness of this star by as much as 3 to 4 magnitudes along with a substantial X-ray flux measured over the years by several orbiting observatories, Ross 154 is considered to be an active star despite is small size. This activity coupled with a short three-day period of rotation strongly suggest that Ross 154 is a young object less than one billion years old. In many ways it is similar to other young nearby red dwarfs such as the somewhat smaller Wolf 359 (see “The Real Wolf 359”).

The Red Dots campaign will use ESO’s exoplanet hunter to look for Earth-like planets around some of our nearest stellar neighbors: Proxima Centauri, Barnard’s Star and Ross 154. (ESO/Red Dots)

Like Proxima Centauri and Barnard’s Star, Ross 154 has been part of the HARPS RV survey of M dwarf stars. A total of eight RV measurements in the initial survey analysis of 2013 by Bonfils et al. showed a periodicity of 2.89 days but the false alarm probability was far too high to be considered a reliable detection and it now seems to be related to the star’s three-day rotation period. As always, more data will be required to sort out what is going on with Ross 154 and detect exoplanets through the natural noise especially exoplanets with orbital periods of a couple of weeks or more which would cover the HZ and beyond.

 

The Red Dots Campaign

At the heart of the new Red Dots campaign is a hundred-day observing run using HARPS starting on June 21, 2017. The goal is to obtain spectra of the three target stars on a total of 90 nights, weather permitting, which can then be used to derive precision RV measurements. This high-cadence data set will be ideal for resolving the orbital period of the second suspected exoplanet orbiting Proxima Centauri as well as search Barnard’s Star and Ross 154 for possible exoplanets. These new data will be analyzed together with archived RV measurements from HARPS and UVES.

The ESO 3.6m Telescope equipped with HARPS that will be collecting radial velocity data for the Red Dots campaign. (ESO/H.H.Heyer)

As was done during the 2016 Pale Red Dot campaign, nearly simultaneous multiband photometry will be acquired of the three target stars starting on June 15. The 0.4-meter ASH2 (Astrograph for the South Hemisphere II) telescope at the San Pedro de Atacama Celestial Explorations Observatory in Chile along with selected one-meter telescopes of LCOGT.net (Las Cumbres Observatory Telescope network) will be used just as they were in the earlier Pale Red Dot campaign. In addition, LCOGT.net’s 0.4-meter telescopes along with the one-meter instruments located in Australia (Siding Spring Observatory), South Africa (Sutherland), Spain (Tenerife, Teide Observatory), Chile (Cerro Tololo Observatory), and United States (McDonald Observatory in Texas and Haleakala Observatory in Hawaii) will be participating in the effort.

Instruments like this LCOGT.net one-meter telescope, which were used during the Pale Red Dot campaign, will also be employed in the new Red Dots campaign to make photometric measurements. (LCOGT)

In addition to these facilities, the T90 telescope of the Sierra Nevada Observatory in Granada, Spain, the 0.8-meter Telescopi Joan Oró (TJO) at Observatori Astronòmic del Montsec (OAdM) in Lleida, Spain and five remotely controlled 0.4-meter Meade LX200 telescopes located at the Bayfordbury Observatory in Hertfordshire, UK will be employed to gather supporting photometric data. As part of a public outreach effort, the Red Dots campaign has also enlisted help of the American Association of Variable Star Observers (AAVSO) so that amateur astronomers can participate in the photometric monitoring of these target stars (for those interested in participating, see this AAVSO Alert Notice). The Red Dots team is especially interested in obtaining nearly continuous 24-hour a day V-band photometry of Ross 154 to track its activity over its 3-day rotation period and AAVSO participation would help fill the inevitable gaps in coverage from the larger participating observatories.

Modest facilities like the OAdM in Lleida, Spain will also be contributing photometric data to support the Red Dots campaign. (OAdM via Red Dots)

Unlike the Pale Red Dot campaign whose data were not publicly revealed until the final analysis was published following peer review, the Red Dots campaign is being run as an open notebook science experiment – the practice of making the entire primary record of a research project publicly available online as it is recorded. The public will have the opportunity to read updates of what is being observed and discussion of the results on the Red Dots website. According to the ESO announcement, however, “any observations presented during this time will of course be preliminary only and they must not be used or cited in refereed literature. The team will not produce conclusive statements, nor claim any finding until a suitable paper is written, peer-reviewed and accepted for publication.”

If the earlier campaign with Proxima Centauri is any guide, the Red Dots campaign should allow us a chance to watch the exoplanet discovery process unfold over the course of this summer and see results formally published around the beginning of 2018. Perhaps we will finally resolve the question of whether Proxima Centauri b has siblings as well as if Barnard’s Star and Ross 154 are accompanied by exoplanets (possibly potentially habitable).

 

Follow Drew Ex Machina on Facebook.

 

Related Reading

“Habitable Planet Reality Check: Proxima Centauri b”, Drew Ex Machina, August 29, 2016 [Post]

“The Search for Planets Around Proxima Centauri”, Drew Ex Machina, February 23, 2015 [Post]

“The Hubble Space Telescope and the Search for Faint Extrasolar Companions”,  SETIQuest, Volume 3, Number 2, pp. 1-9, Second Quarter 1997 [Article]

“The Search for Planets Around Barnard’s Star”, Drew Ex Machina, April 23, 2015 [Post]

 

General References

Guillem Anglada-Escudé et al., “A terrestrial planet candidate in a temperate orbit around Proxima Centauri”, Nature, Vol. 536, No. 7617, pp. 437-440, 25 August, 2016

X. Bonfils et al., “The HARPS search for southern extra-solar planets XXXI. The M-dwarf sample”, Astronomy & Astrophysics, Vol. 549, ID A8, January 2013

Jieun Choi et al., “Precise Doppler Monitoring of Barnard’s Star”, The Astrophysical Journal, Vol. 764, No. 2, Article id. 131, February 2013

M. Endl and M. Kürster, “Toward detection of terrestrial planets in the habitable zone of our closest neighbor: Proxima Centauri”, Astronomy and Astrophysics, Vol. 488, No. 3, pp.1149-1153, September 2008

P. Kervella, F. Thévenin and C. Lovis, “Proxima’s orbit around Alpha Centauri”, Astronomy & Astrophysics, Vol. 598, ID L7, February 2017

“VLT to Search for Planets in Alpha Centauri System – ESO Signs Agreement with Breakthrough Initiatives”, Organization Release ESO 1702, January 9, 2017 [Release]

“Photometry requested for Red Dots campaign”, AAVSO Alert Notice 583, June 16, 2017 [Notice]

“Red Dots: The Live Search for Terrestrial Planets around Proxima Centauri Continues”, ESO Announcement 17036, June 19, 2017 [Announcement]