Beta Pictoris is a star in the constellation of the Painter. The star's name follows the convention of Bayer nomenclature, being without a traditional name, and with a magnitude of +3.86, it is the second brightest star in its constellation, after Alfa Pictoris. Its distance from the solar system, calculated from measurements by the Hipparcos satellite, is equal to 63.4 light years.

Beta Pictoris is a young white main sequence star of spectral class A6V, with an estimated age of no more than 20 million years. It has a luminosity nine times that of the Sun and a surface temperature of 8050 K, while its mass and radius are approximately 75% greater than those of the Sun. It is part of the Beta Pictoris Association, to which it gives its name, a young group of stars with a common motion in space and the same origin.

The star shows an excess of infrared emission compared to other stars of its class, caused by the large amount of dust and gas present around it.

Detailed observations have revealed the presence of a large circumstellar disk, generally considered to be a protoplanetary disk; it was the first circumstellar disk to be observed around another star.

In addition to the presence of belts of planetesimals and cometary activity, there are indications of the presence of planets within the dust disk, and that the process of planetary formation itself is still ongoing.

In November 2008, ESO announced the presence of a giant planet based on infrared observations made with the VLT telescope in 2003. The planet, Beta Pictoris b, is the closest to its star that has ever been directly imaged: the distance from the star (8 AU) is a little less than that which divides Saturn from the Sun. A second planet, also a gas giant, was discovered in 2019, Beta Pictoris c.

Beta Pictoris is bright

Beta Pictoris is located in the Southern Hemisphere, at an eccentricity of −51° 05′ (at that time J2000.0); It can theoretically be seen on the southern horizon in the winter months also from the African coast of the Mediterranean Sea, as well as from the southern states of the United States, despite its geographic location in those geographical areas.

Very low on the horizon and somewhat difficult to see. From locations such as Melbourne, Australia, or Wellington, New Zealand, it appears circumpolar, dominating the austral summer night sky.

With an apparent magnitude of 3.86, it is not a prominent star in the sky; However, they can also be observed from small urban centres, as long as they are not affected by excessive light pollution. What facilitates its search in the sky is its relative proximity to Canopus, the second brightest star in the night sky, which is located a few degrees northwest of it. However, its great distance from the ecliptic means that luminous objects in the solar system are always far from the star in the celestial dome.

The best period for observing it is during the Australian summer months, from November to April, while in the northern tropical belt, its observation is limited to the northern winter months.

Beta Pictoris and Spatial motions

The distance to Beta Pictoris and many other stars was measured by the Hipparcos satellite, using the trigonometric parallax method: for Beta Pictoris the parallax was measured at 51.87 milliarcseconds, then corrected to 51.44 mas when the data were analyzed again taking into account some systematic errors. This value corresponds to a distance of 63.4 + 0.1 light years from Earth.

Hipparcos also measured the proper motion in the sky of Beta Pictoris, which moves eastward at 4.65 mas per year, and northward at a rate of 83.10 milliarcseconds per year.

The radial velocity of a star indicates whether it is moving towards or away from the Sun, and is measured through the Doppler effect; in the case of Beta Pictoris the radial velocity is approximately +20 km/s, where the positive sign indicates that the star is moving away from the solar system.

Many other stars share the same movement in space because they share the same origins and the same gas cloud from which they formed, such as the other stars that are part of the Beta Pictoris Association.

Beta Pictoris and Physical Properties

Brightness and variability

Beta Pictoris is a white main sequence star of A6V spectral type, with a surface temperature of 8052 K. Being a class A star, like Sirius and Vega, Beta Pictoris is brighter than the Sun; combining the apparent magnitude of 3.861 with a distance of 19.44 parsecs gives an absolute magnitude of 2.42, compared to the absolute magnitude of the Sun which is 4.83. This corresponds to a visible brightness 9.2 times higher than that of the Sun.

In 2003, photometric monitoring of the star revealed brightness variations of 1-2 thousandths of magnitude, over periods of about 30 and 40 minutes.

Some studies on the radial velocity of Beta Pictoris have also revealed variability: there are pulsations with two periods, one of 30.4 minutes and one of 36.9 minutes. Due to these small variations, the star is classified as a Delta Scuti variable.

Mass, radius, and rotation

Based on stellar evolutionary models and the properties of the star obtained from observations, the mass of Beta Pictoris is estimated to be between 1.7 and 1.8 times that of the Sun.

The angular diameter of the star was measured using the interferometer of the Very Large Telescope and was found to be 0.84 mas, while with the same instrument De Folco et al. in 2004 measured an angular diameter equal to 0.849 + 0.060 mas. Combining this value with the distance of 63.4 light years calculated by Hipparcos results in a radius 77% greater than that of the Sun.

The rotation speed of Beta Pictoris has been measured to be at least 130 km/s. Since this value is determined by measuring the radial velocity, this is a lower limit of the rotation speed: the measured value is sin v (i), where i represents the inclination of the star's rotation axis concerning the line of sight in the direction of the Earth.

Under the assumption that the equatorial plane of Beta Pictoris is viewed from the Earth edge-on, the rotation period would be approximately 16 hours, which is significantly less than that of the Sun (609.12 hours).

Age and education

The presence of significant amounts of dust around the star suggests that the system is quite young. There was also discussion about whether to consider Beta Pictoris already main-sequence or still a pre-main-sequence star.

When the distance to the star was measured by Hipparcos, astronomers realized that Beta Pictoris was further away than previously thought, and was therefore actually brighter than previously thought. Taking the Hipparcos data into account, it was found that Beta Pictoris is located near the zero age of the main sequence and was no longer a pre-sequence star.

Beta Pictoris is part of the group that bears its name, the Beta Pictoris Association, and analysis of the other stars in the group suggests that they are about 12 million years old. Taking into account a certain margin of uncertainty, the age of Beta Pictoris can be between 8 and 20 million years.

Beta Pictoris may have formed near the Scorpius-Centaurus Association.

The collapse of the gas cloud that led to the formation of Beta Pictoris may have been triggered by the shock wave of a supernova: the star that exploded as a supernova may have been a former companion of HIP 46950, which is currently a star fugitive.

Tracing the path of HIP 46950 on the contrary shows that it would have been in the vicinity of the Scorpius-Centaurus association about 13 million years ago.

Beta Pictoris and Planetary System

Circumstellar disk

An excess of infrared radiation from Beta Pictoris was detected by the IRAS probe in 1983. Together with Vega, Fomalhaut, and Epsilon Eridani, it was one of the first four stars in which an excess of infrared radiation was detected. The hypothesis was proven in 1984 when Beta Pictoris became the first star around which a circumstellar disk was directly observed.

The debris disk around Beta Pictoris is seen edge-on from Earth, and is asymmetric: in a northeast direction, it was observed at 1835 AU from the star, while in a southwest direction at 1450 AU away from Beta Pictoris.

In 2006, observations with the Hubble Space Telescope revealed the presence of a secondary dust disk extending up to 130 AU from the star, tilted 5° relative to the primary.

Astronomers hypothesize that the secondary disk may have formed due to a massive planet in an orbit inclined to the main disk. The planet would have removed matter from the primary disk, causing it to flow into an orbit aligned with the same planet.

In 2011, the disk around Beta Pictoris was observed for the first time by an amateur astronomer, New Zealander Rolf Olsen, who captured the image with a 10-inch Newtonian reflector and a modified webcam.

Observations carried out in 2004 highlighted the presence of an internal ring of matter containing silicates at a distance of 6.4 AU from the star. Silicate-based matter has also been detected at a distance between 16 and 30 AU from the star. The lack of matter between 6.4 and 16 AU may be an indication of a massive planet orbiting in this region.

Belts of planetesimals

In 2003, observations of the inner region of the Beta Pictoris system with the Keck telescope revealed the presence of rings of matter.

Rings of matter were detected at approximately 14, 28, 52, and 82 AU from the star, with a plane inclined to the main debris disk.

The shape of the dust disk 100 AU from the star suggests that the dust grains in this region could be produced by a series of collisions of planetesimals with radii of about 180 kilometers.

After the initial impact, the debris undergoes further collisions in a process called a collisional cascade. Similar processes have been hypothesized in the dust disks around Fomalhaut and AU Microscopii.

The spectrum of Beta Pictoris shows strong short-term variability that is thought to be caused by material falling towards the star. It has been suggested that this matter derives from small comets placed in orbits close to the star, at a distance where they begin to evaporate.

However, it is thought that the infalling bodies are probably composed of mixed dust and an ice core, with a crust of refractory material, and not completely frozen like comets.

These objects may have been perturbed by the gravitational influence of a planet in a slightly eccentric orbit around Beta Pictoris, at a distance of about 10 AU from the star.

Planet b

The presence of a giant planet, probably responsible for the anomalies observed in the shape of the dust disk, was confirmed using the VLT telescope, with which the planet was directly observed in 2009. The results of the discovery were exposed in a published study In 2010.

If this planet were in the solar system it would orbit more or less at the distance of Saturn. It is the exoplanet closest to its star.

Beta Pictoris and discoveries

The radial velocity method used to discover most exoplanets is not well suited to studying A-type stars like Beta Pictoris, and its young age makes the study even more difficult.

In the case of Beta Pictoris, this method is only sufficient to exclude hot Jupiters with more than 2 times the mass of Jupiter at a distance less than 0.05 AU from the star.

Planets orbiting 1 AU away with less than 9 Jupiter masses would have no major effect on the star's radial velocity. So, to find planets in the Beta Pictoris system, astronomers looked for the planet's effects on the circumstellar environment. The space between the two belts of planetesimals at 6.4 and 16 AU appears clear of material, and this suggests the presence of a massive planet about 10 AU away from the star.

A planet at that distance would also explain the fall of bodies that evaporate a short distance from Beta Pictoris, as well as the orbital inclination of the rings of matter in the inner circumstellar disk.

However, the planet discovered and directly observed alone cannot explain the structure of the belts of planetesimals at 30 and 52 AU from the star.

These belts could be related to smaller planets 25 and 44 AU away, having a mass of about 0.5 and 0.1 times that of Jupiter, respectively. It is also possible that the outer disk rings at 500-800 AU are indirectly affected by the presence of these putative planets.

Beta Pictoris b was observed at an angular distance of 411 mas from the star, which corresponds to a real distance of approximately 8 AU.

According to theoretical models of planetary evolution, the planet is predicted to be about 8 Jupiter masses, still cooling after its formation, with a temperature between 1400-1600 K. The semi-major axis of the planet's orbit is 8-9 UA and its orbital period is 17-21 years.

A transit of the planet would have been observed in 1981: if indeed that was a true planetary transit, the radius of the planet could be estimated at 2-4 times that of Jupiter, higher than the models predicted by scientists. This could indicate that the planet is surrounded by a large ring system or a forming moon.

Observations at the Very Large Telescope (VLT) carried out in 2014 by a team of Dutch astronomers allowed, for the first time, to determine the rotation period of an exoplanet: Beta Pictoris b rotates on itself in only eight hours, faster than any planet in the solar system.

The speed at the planet's equator is approximately 100,000 kilometers per hour. However, this result confirms, in broad terms, what happens in the solar system, that is, that massive planets rotate on themselves more rapidly than less massive planets.

A second planet, a gas giant with a mass eight or nine times that of Jupiter and with an orbit of about 1200 days, was discovered in 2019 by studying data obtained with the HARPS spectrograph at ESO.

It orbits more internally than planet b, over 3.35 years, its radius has been estimated at 1.2 rJ, and its temperature at 1250+50 K.