Young Sun-like Star Shows a Magnetic Field Was Critical for Life on the Early Earth

CfA Release 2016-06. For release: Wednesday, March 16, 2016

Published to ApJL Letters, astro-ph:1603.03937v1 preprint; CfA-Smithsonian Harvard Press Release

Figure 1. In this artist’s illustration, the young Sun-like star Kappa1 Ceti is blotched with large starspots, a sign of its high level of magnetic activity. New research shows that its stellar wind is 50 times stronger than our Sun’s. As a result, any Earth-like planet would need a magnetic field in order to protect its atmosphere and be habitable. The physical sizes of the star and planet and distance between them are not to scale. [Credit: M. Weiss/CfA]

Nearly four billion years ago, life arose on Earth. Life appeared because our planet had a rocky surface, liquid water, and a blanketing atmosphere. But life thrived thanks to another necessary ingredient: the presence of a protective magnetic field. A new study of the young, Sun-like star Kappa1 Ceti shows that a magnetic field plays a key role in making a planet conducive to life.

“To be habitable, a planet needs warmth, water, and it needs to be sheltered from a young, violent Sun,” says lead author Jose-Dias Do Nascimento of the Harvard-Smithsonian Center for Astrophysics (CfA) and University of Rio G. do Norte (UFRN), Brazil.

Kappa1 Ceti, located 30 light-years away in the constellation Cetus, the Whale, is remarkably similar to our Sun but younger. The team calculates an age of only 400-600 million years old, which agrees with the age estimated from its rotation period (a technique pioneered by CfA astronomer Soren Meibom). This age roughly corresponds to the time when life first appeared on Earth. As a result, studying Kappa Ceti can give us insights into the early history of our solar system.

Figure 2. This computer model shows the magnetic field lines of the star Kappa Ceti as gray lines looping out from the star’s surface. This young, Sun-like star generates a stellar wind 50 times stronger than our Sun’s. As a result, any potentially habitable planet would need a magnetic field to protect its atmosphere. [Credit: TCD/Vidotto. CfA/do Nascimento et al. ApJL Letters]

Like other stars its age, Kappa1 Ceti is very magnetically active. Its surface is blotched with many giant starspots, like sunspots but larger and more numerous. It also propels a steady stream of plasma, or ionized gases, out into space. The research team found that this stellar wind is 50 times stronger than our Sun’s solar wind.

Such a fierce stellar wind would batter the atmosphere of any planet in the habitable zone, unless that planet was shielded by a magnetic field. At the extreme, a planet without a magnetic field could lose most of its atmosphere. In our solar system, the planet Mars suffered this fate and turned from a world warm enough for briny oceans to a cold, dry desert.

The team modeled the strong stellar wind of Kappa1 Ceti and its effect on a young Earth. The early Earth’s magnetic field is expected to have been about as strong as it is today, or slightly weaker. Depending on the assumed strength, the researchers found that the resulting protected region, or magnetosphere, of Earth would be about one-third to one-half as large as it is today.

“The early Earth didn’t have as much protection as it does now, but it had enough,” says Do Nascimento.

Figure 3. In this artist’s illustration, the young Sun-like star Kappa1 Ceti is blotched with large starspots, a sign of its high level of magnetic activity. New research shows that its stellar wind is 50 times stronger than our Sun’s. As a result, any Earth-like planet would need a magnetic field in order to protect its atmosphere and be habitable. The physical sizes of the star and planet and distance between them are not to scale. [Credit: Rogério Melo (Edufrn/UFRN)]

Kappa1 Ceti also shows evidence of “superflares” – enormous eruptions that release 10 to 100 million times more energy than the largest flares ever observed on our Sun. Flares that energetic can strip a planet’s atmosphere. By studying Kappa Ceti, researchers hope to learn how frequently it produces superflares, and therefore how often our Sun might have erupted in its youth.

This research has been accepted for publication in The Astrophysical Journal Letters and is available online.

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Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

For more information, contact:

-Christine Pulliam
Media Revelations Manager
Harvard-Smithsonian Center for Astrophysics
+1 617-495-7463

-Dr. José-Dias do Nascimento Jr.
Astrophysicist, lead author
Harvard-Smithsonian Center for Astrophysics
+55 84 99992-6288

Figure 4. Observations taken with the 2.0-meter Bernard Lyot Telescope at Pic du Midi Observatory in France show that Kappa Ceti is a Sun-like star with an age of 400 – 600 million years.[Credit: Jose-Dias do Nascimento]



When, How and Under What Conditions Life Arose on Earth?

If an alien astronomer would report our home solar system he would probably make an incorrect conclusion, that from all the eight planets orbiting the Sun, two were suitable for life. However only one harbor life. Mars and Earth have warm surface enough for liquid water and while Earth is blessed with a liquid ocean, Mars is dry and dead. Why?

The transition from an inhabitable young planet to the living planet remains one of the main enigmas faced by science. To answer this question scientists from different fields dedicated entire lives and the answer essentially comes from the study of the young Sun. Since we can not go back in time, a solution is to study a young star with mass, age and chemical abundance mimicking perfectly the Sun at that time when life arose on earth.


The magnetic field of the Young Sun and the Goldilocks zone

Habitable zone is also called the Goldilocks zone, a metaphor of the children's fairy tale. A habitable zone define a distance from a star where liquid water could exist on an orbiting planet. Actually, we have a scientific method to determine whether there is life on another planet and this method is based just on the distance and does not take into account the magnetic interaction between the young planet and the young star.

Our Sun is constantly hurling deadly radiation and particles, thus is a key factor for understanding the origin, evolution and support. The particle and magnetic environment define the type of interactions between the star and the planet. In the case of magnetized planets, such as the Earth that developed a magnetic field at least four billion years ago, their magnetic fields act as obstacles to the stellar wind, deflecting it and protecting the upper planetary atmospheres and ionospheres against the direct impact of stellar wind plasma and high-energy particles.

To provide the magnetic field strength at that time when life arose on earth, a team of astrophysicists led by José-Dias do Nascimento (Scientist at the Harvard Smithsonian Center for Astrophysics, CfA and professor at UFRN University, Natal Brazil) has observed with spectropolarimetry a genuine young Sun’s proxy, Kappa1 Ceti and reconstructed its large-scale surface magnetic field to derive the magnetic environment, stellar winds and particle flux permeating the interplanetary medium.

















Figure 5. Large-scale magnetic topology of κ1 Cet at different rotation phases indicated in the top right of the panel. The animated gif shows the inclination of field lines over stellar surface, with red and blue arrows depicting positive and negative field radial component values, respectively. [ApJL/CfA-Harvard/UFRN/do Nascimento et al. 2016]


Why Kappa1 Ceti ?

Among the solar proxies, Kappa1 Ceti (κ1 Cet, HD 20630, HIP 15457) a nearby young solar twin G5 dwarf star with V = 4.85 and age from 0.4 Gyr to 0.6 Gyr stands out potentially having a mass very close to solar and age of the Sun when the window favorable to the origin of life opened on Earth around 3.8 Gyr ago or earlier. This corresponds to the period when favorable physicochemical and geological conditions became established and after the late heavy bombardment. As to the Sun at this stage, Kappa1 Ceti radiation environment determined the properties and chemical composition of the close planetary atmospheres, and provide an important constraint of the role played by the Earth’s magnetospheric protection at the critical time at the start of the Archean epoch, when life is thought to have originated on Earth. This is also the epoch when Mars lost its liquid water inventory at the end of the Noachian epoch some 3.7 Gyr ago. Study based on Kappa1 Ceti can also clarify the biological implications of the high-energy particles at this period. Such a study requires careful analysis based on reasonably bright stars at this specific evolutionary state. Kappa1 Ceti is a unique laboratory for understanding the activity of the Sun when life evolved on the Earth. Perfect for understanding how the early magnetic field affected the young earth back then, also for understanding how a larger particle and X-ray fluxes from the larger corona and a larger UV flux from a stronger activity could affect the evolution of life.
















"Studying Kappa1 Ceti, which is a proxy of the young Sun, we can infer what were the living conditions of the young Earth from the perspective of an interaction between the planet and the central star at the age when life was emerging on Earth”, say Dr. José-Dias.

“Kappa1 Ceti is considered an excellent candidate to host terrestrial planets, not yet further detected. Astronomers already knew that young stars were much more magnetically active than our Sun, however now we measured the intensity of the magnetic field at this critical period for a genuine proxy of the Sun with an age at that time when life arose on earth.”

The complex magnetic-field topology of Kappa1 Ceti gives rise to non-uniform directions and strengths along a possible planetary orbit. Our stellar wind model for Kappa1 Ceti shows a mass-loss rate factor 50 times larger than the current solar wind mass-loss rate, resulting in a larger interaction between the stellar wind and a hypothetical young-Earth like planet.


How old is Kappa Ceti ?

It is just about the one of most difficult questions you can ask about a star. For Kappa1 Ceti we used different complementary approach to determine its age. We measured the average surface Prot from light curves obtained with MOST (Microvariability and Oscillations of Stars). MOST continuously observed Kappa1 Ceti for weeks at a time providing a Prot. We extract Prot and this value allowed us an independent (from classical isochrone) age derivation of Kappa1 Ceti using gyrochronology. The gyrochronology age of Kappa1 Ceti that we derive is around 0.6 - 0.7 Gyr, consistent with the predictions from other team and ages determined from evolutionary tracks.

Spectropolarimetric data of Kappa1 Ceti were collected with the NARVAL spectropolarimeter at the 2.0-m Bernard Lyot Telescope (TBL) of Pic du Midi Observatory, France, located at 2877 m elevation. NARVAL comprises a Cassegrain-mounted achromatic polarimeter and a bench-mounted cross-dispersed echelle spectrograph. In polarimetric mode, NARVAL has a spectral resolution of about 65,000 and covers the whole optical domain in one single exposure, with nearly continuous spectral coverage ranging from 370 nm to 1000 nm over 40 grating orders.

Figure 6.Timelapse Video of the 2.0-meter Bernard Lyot Telescope at Pic du Midi Observatory in France.[Credit: Romain Montaigut]


The team is composed by

  • José-Dias do Nascimento Jr, Astrophysicist, lead author - Scientist at Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA; Professor at Departamento de Física, Univ. Federal do Rio G. do Norte, UFRN, Brazil;


  • S. Meibom - Scientist at Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA;
  • A.A. Vidotto - Scientist at Trinity College Dublin, College Green, Dublin 2, Ireland;
  • P. Petit, - Scientist at Univ. de Toulouse, UPS-OMP, Inst. de R. en Astrop. et Planetologie, France; 5CNRS, IRAP, 14 Av. E. Belin, Toulouse, France;
  • C. Folsom - Scientist at Univ. Grenoble Alpes, IPAG, Grenoble, France;
  • S. Marsden - Professor at Computational Engineering and Science Research Centre, University of Southern Queensland, Toowoomba, Australia;
  • J. Morin - Scientist at LUPM-UMR5299, Univ. de Montpellier, France;
  • G. F. Porto de Melo - Professor at Observatório do Valongo, UFRJ, L do Pedro Antonio, RJ, Brazil.
  • S.V. Jeffers - Scientist at fur Astrophysik, G.-August-Univ.,Goettingen, Germany.
  • I. Ribas - Scientist at Institut de Ciències de l’Espai (CSIC-IEEC), Carrer de Can Magrans, s/n, Campus UAB, Bellaterra, Spain;
  • M. Castro - Professor at Univ. Federal do Rio G. do Norte, UFRN, Dep. de Física, Brasil;
  • E. Guinan - Professor at Univ. of Villanova, Astron. Department, PA ,Pennsylvania, US;

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