Gliese 581
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| Gliese 581 | |
 En konstnärs rendering av Gliese 581 | |
| Observationsdata Epok: J2000 | |
|---|---|
| Stjärnbild | Vågen |
| Rektascension | 15t 19m 26s |
| Deklination | -07° 43′ 20″ |
| Skenbar magnitud () | 10,56 till 10,58 |
| Stjärntyp | |
| Spektraltyp | M3V |
| Astrometri | |
| Avstånd | 20,37 lå (6,26 pc) |
| Absolut magnitud () | 11,6 |
| Detaljer | |
| Massa | 0.31 MSun[1] M☉ |
| Radie | 0.29 rSun[1] R☉ |
| Temperatur | 3480 K |
| Ålder | 7 till 11 miljarder år |
| Andra beteckningar | |
| HO Librae, HIP 74995, Gl 581, BD-07° 4003 | |
Gliese 581 är en stjärna av sorten röd dvärg cirka 20 ljusår från Solen, med ungefär en tredjedel så stor massa som denna. Den ligger i stjärnbilden Vågen (Libra på latin). I omloppsbana runt Gliese 581 finns planeten Gliese 581 b, en planet av Neptunus storlek, och Gliese 581 c, en relativt nyupptäckt planet som eventuellt kan ha vatten och hysa liv. Uppgift finns även om förekomst av en planet Gliese 581 d.
Den 21 april 2009 upptäcktes en fjärde planet i systemet, Gliese 581 e. Denna planet har en minsta massa på 1.7 jordmassor.
I slutet av september 2010 upptäcktes ytterligare två planeter, Gliese 581 g och Gliese 581 f. Gliese 581 g är då den första planeten som upptäckts i den beboeliga zonen. Planetens rotation är också synkroniserad på så sätt att den visar samma sida mot stjärnan hela tiden.
Den 27 november 2012 meddelade ESA att man hittat en fragmentskiva i omlopp runt Gliese 581 med minst tio gånger fler kometer än solsystemet.
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After more than four years of observations using the most successful low-mass exoplanet hunter in the world, the HARPS spectrograph attached to the 3.6-metre ESO telescope at La Silla, Chile, astronomers have discovered in this system the lightest exoplanet found so far: Gliese 581e (foreground) is only about twice the mass of our Earth. The Gliese 581 planetary system now has four known planets, with masses of about 1.9 (planet e, left in the foreground), 16 (planet b, nearest to the star), 5 (planet c, centre), and 7 Earth-masses (planet d, with the bluish colour). The planet furthest out, Gliese 581d, orbits its host star in 66.8 days, while Gliese 581 e completes its orbit in 3.15 days.
Summary
SVG image of the Gliese 581 system orbits. Plot of the orbits of the Gliese 581 system, including the unconfirmed planet g.
Orbit data taken from Vogt et al. (2010), "The Lick-Carnegie Exoplanet Survey: A 3.1 M⊕ Planet in the Habitable Zone of the Nearby M3V Star Gliese 581"
The configuration is given in Table 2. The 6-planet solution quoted is for circular orbits, thus eccentricities are not represented.
Assumptions:
- It is assumed that the system is coplanar. This cannot be constrained by the radial velocity observations used to detect the planets in this system, but our solar system's planets are nearly coplanar, and both the systems of PSR B1257+12 (planets B and C) and Epsilon Eridani (planet b and the circumstellar disk) are coplanar, see Konacki and Wolszczan (2003), The Astrophysical Journal 591, L147–L150 "Masses and Orbital Inclinations of Planets in the PSR B1257+12 System" and Beust et al. (2006), The Astrophysical Journal 132, 2206–2218 "The Extrasolar Planet ɛ Eridani b: Orbit and Mass respectively, so this assumption is at least reasonable.
- The direction of rotation around the star is assumed to be the same for each planet. Again, this cannot be constrained by the current radial velocity observations, but is a reasonable assumption since this is true for the major planets in our own solar system, and is predicted by current theories of planet formation. Under these first two assumptions, the orientation of the orbits relative to each other is correct as depicted in this diagram, however their actual orientation in 3D space with respect to external reference points (e.g. the Sun, the galactic centre) is unknown, hence such directions are not indicated on the diagram.
- A further assumption is that the true masses of the planets are small compared to that of the star. If the masses were comparable to that of the star, the actual semimajor axis corresponding to the measured orbital period would be greater. This assumption is supported both by probability (the chance that we are observing a system which has sufficiently low inclination for this to be relevant is very low) and by dynamical stability arguments which indicate that the planets cannot have true masses much more than 1.6 times their minimum masses (Mayor et al., 2009). This effect can therefore safely be neglected.
Interpreting the diagram
The planetary orbits are drawn in orthographic projection, as viewed from directly above the plane of the system so that the orbital direction is anticlockwise. The position of each planet in its orbit is given for the specified epoch, given in the Vogt et al. (2010) paper.
While the orbits are drawn to the correct scale, the star and the planets themselves are not shown to scale as they would be too small to see on the scale of this diagram. In addition, the true radii of the planets themselves are currently unknown as no direct observation of the planets has yet been made.The orbits of planets in the Gliese 581 system are compared to those of our own solar system. The Gliese 581 star has about 30 percent the mass of our Sun, and the outermost planet is closer to its star than the Earth is to the Sun. The 4th planet, G, is a planet that could sustain life. Note: Planet sizes not to scale.